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Vehicle efficiency upgrades: 30+ MPG in 2.5ton commercial truck

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[note:  new video has been added on step 1!]
[note 2: Most of what I write isn't relevant to Instructables.  My main blog is here: biodieselhauling.blogspot.com]

[note 3: Also save energy at home.  See my energy guide: instructables.com/id/Not-your-average-save-energy-advice-use-less-en/ ]

Go 50-100% farther on a tank of fuel.

Due to its size and weight this truck is considered a commercial vehicle and is exempt from even light-truck CAFE standards. Even so, with the modifications I have made, I am getting higher mileage than CAFE standards for 2009 cars.

I read an article in Mother Jones Magazine about Wayne Gerdes, mileage champion, and was inspired. I have read that people tend to get between 10 and 16mpg on average in the same truck I have. I was getting around 15. After the mods described here, on a recent tank I got 30.28mpg.

The best thing to do is to not drive at all. Ride a bike, take the train, carpool.
If you do drive, buy the absolute smallest car you can. If you only need a big vehicle occasionally, rent one.

I bicycle or use my 70mpg 250cc motorcycle for personal transport, but my truck still gets a lot of miles, so I wanted to make it as fuel efficient as possible.
I use this truck for work, moving up to 3 tons of soil or broken concrete, entire 1 bedroom apartments worth of belongings (including furniture) in a single trip, etc.
I also need something with enough power to move my 7500lb RV trailer (which is also my home - a very efficient way to live: I use as much electricity in a month as the average American home uses in one day)

Most of these steps could be done to any vehicle, increasing mileage from 50% to 100% or more.
 
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Step 1: The cheapest, and most important step

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Modify driving habits

Single most significant difference, in any vehicle: "Adjust the nut behind the wheel"
(Drive more efficiently.)

Stay at (or below) the speed limit.
(Remember, you only save 7 seconds per mile going 75mph instead of 65mph, but stopping distance and wind resistance increase exponentially; twice as fast = 4 times the force)
(I have a much more in-depth look at speed at ecomodder.com/blog/slow)

Accelerate gently (Keep RPMs low whenever possible)

Never accelerate towards a stop light. (Every time you brake you are wasting momentum.  Remember this with the rhyme: "if you have to break, you made a mistake)

Never idle. A popular misconception is that it uses more gas to start than to idle. This was true of older (carbureted) engines. This is not true of modern (fuel injected) engines.

More advanced techniques include actually shutting off the engine and coasting as often as practical. (This can change the feel of the brakes and steering, see step 4)

(PLEASE READ the entire instructable and all the comments before commenting about how dangerous it is to coast.)

NOTE: many automatic transmissions can be damaged by coasting with the engine off.
To find out if yours is one of them, consult this list: http://www.motorhomemagazine.com/dinghytowingguide/
If you have an automatic, and your car is on the list, you can shut the engine while moving.
If it is not, do so at your own risk. You may still benefit from shifting into neutral (engine on) when coasting downhill or towards a stop, depending on whether or not your car has automatic deceleration fuel cut off (DFCI) built in (many newer cars do).

Step 2: Improving aerodynamics

This is what I did on my 1983 F-250.
Not everything will be directly applicable to your vehicle. Modify and improvise as appropriate.

-I added an underbelly panel to smooth airflow beneath the vehicle made from Coroplast (corrugated plastic - the stuff cheap signs are made from). It is attached to the front bumper, and about 1/2 way back with cut outs for the wheels to turn, and a cut out for oil changes (and to avoid the hot exhaust manifold).  Then there are smaller pieces anywhere a pocket of air could collect, while letting the transaxel and muffler plenty of space.

-I built some homemade wheel "skirts" (wheel-well covers) to improve aerodynamics. The current  are made of cardboard coated in layers of aluminum tape, and then painted and edged with velcro. I lined the wheel well with velcro, and stuck 'em together, then taped over the seams with clear tape to cover the gaps of a less than perfect measuring and cutting job. These were originally just supposed to be a test, but they held up to a winter of rain and months of hauling, so I have no motivation to replace them.

-The headlights come in a little scoop. I had covers custom cut out of thin Lexan plastic, which is totally clear, scratch resistant, flexible, and nearly indestructible (the same stuff my custom motorcycle windshield is made from). I traced the headlights and brought the tracing to Tap plastics who were able to cut them for me in about 5 minutes. I drilled holes in the corners and screwed them onto the grill so I can easily remove them if a headlight burns out.

-I taped over the front turn signals, (because tape is cheaper than Lexan.)

-I removed the windshield wipers (inc. the entire arm). They come off in a few seconds with no tools. I keep them inside the cab, just in case I get caught in unexpected rain.

-I installed a tonneau cover which, in addition to increasing the trucks aerodynamics, will keep small loads clean, dry, and protected, making it a (semi)legitimate business expense.
I cut triangles out of plywood to mount the cover's rails at an angle, so that it tapers down from halfway up the window to the tailgate. 
Having it angled this way makes it much more aerodynamic (I picked up about 1-2mpg just from tilting the cover), but having it go only 1/2 up the cab instead of joing at the top of the roof means I can still see over it and don't lose any visibility to the rear.  It rolls up so I still have full access to the bed when I need it.

Step 3: Disconnecting extra engine loads: fan, pump, alternator, a/c

A surprisingly large amount of the money you put into the gas tank never makes it to the wheels.  I read somewhere that only about 1% of the energy in gas actually goes to moving the driver.  Roughly 1/2 of that energy just goes into turning the engine.  The law of thermodynamics won't let you recover much of that waste, but you can reduce the parasitic loads between the engine and the wheels.  Every belt driven accessory gets it's power from the gas in the tank, so reducing the accessories will noticeably improve mileage.

-I replaced the stock engine driven hydraulic clutch cooling fan with an electric one.
The fan adds to the battery load, but it is much lighter, so it does not require as much force to turn. Also, it is only needed when driving very slow or idling, as otherwise the movement of the vehicle provides air flow. The stock fan turned at all times, no matter what temperature the engine was at.
This also helps the engine run warmer, which makes it more efficient in cold weather.

-I replaced the belt driven mechanical vacuum pump with an electric one.
This step is only relevant to diesel engines, as gas engines generate vacuum directly (no need for a pump) - unless you coast with the engine off. With an electric vacuum pump the brakes work the same whether the engine is on or off.

-I removed the power steering pump and replaced the steering gear with one meant for manual steering. Even in a truck this size, the only time it is at all difficult is when turning the wheels at a complete standstill. Other than parking in tight quarters, power steering is totally pointless.

All electric power in a car is generated by the alternator, which is powered by the engine, so any reduction in electric draw ultimately reduces drain on the engine.

NOTE: If anyone is inspired to do a similar project (with any vehicle), do not just disconnect the alternator from the battery w/o disconnecting the smaller wires. It will continue to produce current, but since that charge has nowhere to go, the alternator will self-destruct.
Also, diesel trucks tend to have very large batteries. Mine has two. This gives me a lot of reserve power to tap into without draining them too much. With an ordinary car battery you will damage it by cycling it too deeply. Once it finally dies, replace it with a deep-cycle (RV or marine) type battery and you'll be fine.

-The orange (on an '83 F-250) wire running from the external voltage regulator to the alternator controls whether the alternator is charging or idle. If you open the circuit the alternator stops charging. Even though it is still being turned, there is no resistance, it just freewheels. At first I just disconnected the wire, and later I installed a switch so that if the battery ever does run too low, I could charge it with the engine just by flipping the switch.
I used it so little that I eventually just removed the alternator altogether, thereby eliminating the belt and a little bit extra weight.

Since the battery is no longer being charged by the engine, I needed to reduce the electric load as much as possible so the battery will last.
I installed a 15watt solar panel to charge the battery. It sits on the passenger side of the dash board while driving, but when I stop I place it on the roof to get more direct sunlight.  If I drive for several hours (with my 400W stereo system on), the solar panel can fully recharge the batteries in about 2-3 days of full sun.  If I need to drive again sooner than that, I also have an onboard 120v AC charger which plugs into a standard wall outlet, much like a plug-in hybrid.

-I ordered extra bright LED bulbs (from superbrightleds.com) for the taillights and tun signals. They are brighter than stock but draw less power. The original parking/brake/signal lights together use more power than the headlights, at 63 watts (parking) to 177 watts (brake) stock. The LEDs total 5.5 to 20 watts.

-The dash lights alone used 15 watts. When I took apart the dash and instrument gauge, I discovered that the original system deliberately blocked and wasted the majority of light the bulbs put out by covering them with a slightly translucent cover, and then shielding the gauges on 3 sides. Which explained why the dash was always so dim. I broke off the filters, cut away the plastic shielding, and instead built reflectors (out of metal tape) to direct the tiny amount of light of my new bulbs on to the gauges. The result is far brighter than it was, and is all red which is easier on night vision, and uses less than 1 watt for all 8 bulbs together (plus it looks really cool).

-I added a voltmeter so I can monitor the batteries.

-I had been thinking about making a buzzer to remind me if I left the lights on, but then it occurred to me that there is really no situation where I need the lights on and don't have the keys, so while I had the dash apart I also rewired the headlights (and aux driving lights which I also just installed) so that they go off when the key is turned off - now I can't possibly accidentally leave them on and drain the battery.

-If I had air conditioning, I would have removed it.  But I didn't to begin with.  You can stay cooler with beaded seat covers over thin white fabric and heat-blocking lightly-tinted window film, available from any auto parts or hardware store for around $15.  Last summer I painted my roof silver to reflect the sun light and keep it out of the cab.  I also have a small 12volt fan on the dashboard.
If you absolutely can't stand the heat, even with the roof painted silver, heat blocking window tint, beaded seat covers, and a smll fan blowing, you can always roll down the windows just a crack to provide additional ventilation.  Opening them just slightly still hurts your aerodynamics less than rolling them down all the way.
It also helps to wear no more clothing than is legally necessary.

Step 4: Facilitate coasting

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Driving with the engine off provides a couple challenges:

The ignition switch isn't designed for such constant use, but more importantly the power steering and power brakes go out.
(Note: Laws vary from state to state. In CA it is illegal to coast downhill in neutral. It is not illegal to coast with the transmission in gear, but the clutch held in.)

-I installed an electric vacuum pump to run the power brake booster.
These were standard on a number of late 80s American full-size cars, so I was able to pull one from the local wrecking yard. This change removes a drain from the engine (diesels have a belt driven booster pump) and prevents the power brakes from going out whenever I shut the engine. Stopping a 2 1/2 ton truck with just the power of your calf muscle is doable, but can be somewhat challenging.

-I switched to manual steering. This both removes the parasitic drain of the power steering pump from the engine, and means that the steering is not affected by shutting off the engine.

-I wired in a kill switch and remote starter onto the gear shift lever which means I don't need to use the key at all.

The switches I got from the local electronics place are dual purpose: wire them one way and they are momentary open, wire them the other and they are momentary closed. I used one each way, so that one stops fuel to the engine, the other triggers the starter. I mounted them side-by-side in a block of scrap wood, attached to the gear shift with a strip cut out of a steel can lid.
As I had hoped, they definitely make coasting downhill and killing it at red lights much faster, simpler, more precise, and safer, then using the key each time (especially since the ignition on this truck has been very finicky as long as I have had it) which encourages me to do it a lot more often.

If you happen to own a 7th generation (80s) Diesel F-250, and you want to wire in a kill switch or remote starter:
The thick red wire with a green stripe goes to the injector pump (for a kill). Wire the switch normally closed in series with the red wire. Since I didn't have any wire as thick as the one I was tapping into, I used two thinner wires in parallel.
The thin red wire with a blue stripe goes to the ignition relay (for a start). Wire it in parallel to a normally open switch. The other end of the switch goes to 12v positive. Ideally find an accessory (no key off) circuit. Some accessory circuits drop voltage slightly due to their load and therefor aren't strong enough to trigger the relay. I ended up wiring to a constant hot lead, which means the starter can be turned over even with out the key in (not a big deal, because it won't catch)

Step 5: Major changes

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Done:
-Once my tires wore out, I replaced them with low rolling resistance tires.  There aren't many LRR too chose from for large trucks.  With installation they cost $800.  They may well not pay back in terms of fuel used.

-When I can afford it, I will eventually replace the 3 speed (plus extra low) transmission with a 4 spd (plus low). I swapped out my BorgWarner T-18 for a ZF-5.  As expected, having an overdrive gear dramatically increased my highway mileage, from my previous best of 30 all the way up to 38. 

-Some people have made large extensions off the rear of their cars called "boat tails" which cause the rear to taper in at an aerodynamic angle.  There is a guy who gets 90mpg in his Honda Civic. http://www.aerocivic.com/
This would be impractical for me, because of how I use the truck, I am currently experimenting with a way to boattail the truck that doesn't affect its operation or capacity.  I have not yet determined if it helps.

Planned:

-When I can afford it, I might lower the truck about 6 inches.  This will make it more aerodynamic, improve handling, and make it easier to load things onto the tailgate.  Sometimes I do need the extra weight capacity (or ground clearance) so I'll add in air suspension so I can raise it when I need to.

-I won't go through the trouble, but installing a turbo charger increases mileage (assuming you don't waste the extra power on accelerating faster)

-I have heard of people replacing the entire steel truck bed with aluminum to save weight. I am not going to go to that extent either.

-below are some random pictures of what this truck does.  They have nothing to do with this step.

Step 6: Additional steps

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Most of these steps don't apply to my vehicle, but I thought I would pass on some general tips.

-remove all excess weight; things like junk in the trunk, spare tire ('spare in a can' does actually work - or just carry a cell phone), for the truly dedicated, the entire rear seat.

-don't use the AC. Better yet, take it out entirely.

-keep the windows up at highway speeds. Yes, with no AC it will be hot. Use the cars vents, take off your jacket, and deal with being slightly uncomfortable. It helps to have a beaded seat cover, and a small fan that plugs into the cigarette lighter.

-use a ScanGuage or MPGuino. It wires into the car's computer to give you instant mileage and other data.

-the basic recommendations: keep the engine tuned, tires inflated (to the sidewall maximum, or possibly even slightly higher), combine trips so the engine stays warm.  Use a block heater if you have one.

-For more tips, and driving style and mileage mods in general, try:
http://ecomodder.com/

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TorkelW3 months ago

Inspiring! I had no idea you could get such big improvement. Its strange actually that many of your mods were standard up until the 1960s. for example most people here in Sweden used radiator covers in wintertime Since it made the heating work better. I am curious if you have thought about converting to electric? I havent found much info on converting bigger trucks.

Kind regards from Stockholm

JacobAziza (author)  TorkelW3 months ago

Its easier to get a big improvement when you are starting from so low a point!

Electric works better with as light a vehicle as possible, with the limiting factor being batteries. There are a few hybrid trucks and buses (which have plenty of space for huge battery packs) but I think pure electrics in big vehicles is still pretty rare.

I removed the power steering belt from my 1985 Toyota Celica and then did a mpg test over 100s of miles. Result? Absolutely no difference. Never tried an alternator delete though.

roosta6 years ago
you should never shut off the engine whilst moving as it disables the power steering, and in some (especially older cars) also locks the steering rack, so you cant turn. steering locks are security devices fitted to cars to prevent theft.
KaydenST roosta9 months ago

He took out his power steering. Interesting thing about the steering though.

JacobAziza (author)  roosta6 years ago
I don't use the key to shut the engine, so the steering lock never engages. Power steering is really unnecessary at speed. If you drive a truck with no power steering, the only time you notice it is when parallel parking. The power brakes on the other hand.... (an electric vacuum pump for the brakes is my next step)
JacobAziza (author)  JacobAziza6 years ago
electric vacuum pump has been installed. Brakes now function exactly the same whether the engine is on or off.
JacobAziza (author)  JacobAziza6 years ago
Manual steering gear is now also installed
mrcllane6 years ago
Shutting the engine off and costing is a good way to wreck. A lot of vehicles need the engine running for the brakes and steering to work.
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KaydenST mrcllane9 months ago

He said he had electric brakes and he took out the power steering, so it makes no difference.

Forgot to mention using your engine for braking.  When your wheels are driving the engine, it's not burning gas, and you save money on brakes.  Of course, you need a manual transmission for that.

   I coast with the engine off all the time, but safety first (I use the clutch as a failsafe way of coasting):
(1)  Normally, I coast with the clutch pushed in, transmission in gear, and the switch on.  That way if a situation arises, I can pop the clutch, start the engine and be under full power in a split second.
(2) for long, clear coasts, I leave the switch off, with the clutch pushed in & transmission in gear.  That way if I start to lose power steering (and brakes) I can briefly engage the engine (in high gear) to restore the vacuum. 
   Losing vacuum In my hilly topography is rarely a problem: by the time I start to lose steering, I have either slowed too much and need to restart the engine, or have accelerated too much and need to slow down by engaging the engine (which restores the vacuum, even if off).
   (3) I wouldn't do this with an automatic.  
  (4)  I suspect that popping your clutch to start your engine can cause a voltage surge which could burn out some electronics if your voltage regulator is defective (This seems to have happened with one of my cars).

Engine off coasting with an automatic can and usually will damage your transmission and possibly your torque converter. The ATF pump is almost always on the input shaft(from the engine and torque converter) and will not pump unless the engine is running (which is why you can't roll start an automatic (unless you have a car where the ATF pump is on the output shaft as with the old Chrysler 300's)). No fluid flow and the torque converter will drain of fluid and COULD be damaged (though not as likely since it drains when you park the car and shut it off, then refills when you start the engine) and the trans won't get proper lubrication, causing damage. This is why you are not supposed to tow an automatic with wheels on the ground
nerys denswei5 years ago
this is NOT true. you engine is still using gas.

SOME cars in SOME very specific scenarios "cut" the injector flow when a lot of very specific conditions are met.

for example my metro will do this but ONLY if I am over 2500rpm (NEVER) and decelerate with the car in gear.

since I never take it to 2500rpm (I try to max at 2000rpm and cruise at 1600-1700 rpm)

My injectors never turn off unless I turn them off.
JacobAziza (author)  denswei5 years ago
I do similarly to you (except I often coast in neutral, as some hypermilers claim it can wear out the throw out bearing to coast in gear)
but I left out these sort of details to avoid making it overly complex for people new to the idea.

nerys mrcllane5 years ago
My car has manual steering and manual brakes. There is "zero" difference in the car engine off or engine on except the gas pedal don't work anymore.

Thats it. Everything else is 100% identical engine on or off.
Joenavy85 nerys5 years ago
he does make a point though, too many people don't think about how shutting off the engine truly affects their vehicle.(though i doubt that anybody driving a Ferrari 360 Modena is going to shut off their engine to try to increase their mileage). i just put the trans in neutral and coast
yes but with older vehicles (and some newer ones) there is a spot in the ingnition where you can still have steering and brakes (if they aren't power brakes and if they are you have 1 shot with the brakes) so if you are in a wreck while coasting then you are the one at fault
nerys nclark5575 years ago
actually most properly maintained cars with power brakes have enough vac for 2-3 FULL presses of the brake pedal. and even when that runs out the brakes still work just fine they just take a bit of muscle. and all but the 90 pounds little ladies have plenty of leg muscles to work them just fine.

its one thing to have them "go out" on you its another when you know in advance whats about to happen (ie the brakes getting harder)

So unless your lacking on the maintenance you should be just fine.

power steering is nearly meaningless above 5mph.

oh and i forgot about a liitle thing called HANDBRAKES that you can use to stop even if your vehicle has ne engine (found that out when helping my dad tow my bro's  old wrecked car.(he lost it on a icy hill met a power line pole)
 

They need the engine running for power assist. Not to "work". This means they will be harder to use but anyone conscious of this particularly if they are going slow and using proper vehicle spacing should have no problems.
that's true, check out my instructable i made, and it appears the reason why. the instructable, i've read it, a quite time ago, and its quite good, as the aerodynamic features ;)
JacobAziza (author)  mrcllane6 years ago
Please read the entire instructable before commenting. I address both of these issues.
I know of people who died turning the ignition off while driving down hill. It happens every year. Think man, if you’re going to tell people to turn off their car while its moving, don’t you think it would be appropriate to give them a warning ON THE SAME PAGE they may die if they try it…….Step four. pffft
JacobAziza (author)  mrcllane6 years ago
About 45,000 people - over 100 per day - die in car crashes in the US. Auto accidents are the number one cause of death among all people under age 40. Driving is dangerous. Even with power brakes (and seat belts and air bags) driving is one of the most dangerous activities you can do. If you get in a car, you may die. If someone is following my advice, they are first off driving SLOWER than normal. Because force varies with the square of velocity, driving slower has an exponential (technically parabolic) effect on both stopping distances and the force of impact should an accident occur. If you practice hypermileing, it requires you to actually PAY ATTENTION when you are driving. If you are watching your gauges, looking at traffic lights 4 blocks down, judging following distances, and determining what gear to be in every moment, you aren't eating, talking on the phone, and changing the stereo. If you choose to coast engine off without changing to electric brakes (as I did for several months in my 2.5 ton truck) you need to be aware of your vehicle, and of stopping distances. However, I did not point this out because if you drive ANY vehicle, in ANY conditions, you need to be aware of your vehicle and your stopping distances. Since you are driving slower than other traffic, your following distances are already much larger than normal. Since you are trying to avoid wasting momentum, you are driving in such a way as to minimize your need for brakes in the first place. As to power steering, it is the most excessive and wasteful invention ever to have become a standard feature. Since changing my large commercial truck to manual steering the ONLY time I can even tell the difference is when I try to turn the wheel at a complete standstill (to get into tight parking spaces). Even moving at 1 mph makes it easy to turn the wheel. I say if you aren't strong enough to turn a wheel without help from a gas engine, you need to be out getting some exercise instead of driving a car.
I appreciate the effort and some of the stuff you’ve done. But safety first Bro. You’re not fooling anybody with your “exponential of parabolic velocity squared" BS. You should probably just walk.
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JacobAziza (author)  mrcllane6 years ago
lol, "“exponential of parabolic velocity squared" BS"?
Yeah, uh - I didn't make it up. That's Newtonian physics. E=1/2m*v2
Driving slower is safer. You don't have to take my word for it.

"risk of involvement in a casualty crash, relative to the risk for a car traveling at 60 km/h, increased at an exponential rate for free traveling speeds above 60 km/h"
http://casr.adelaide.edu.au/speed/exec.html

“First, the probability of a crash is approximately proportional to the square of the travel speed. Second, in a crash, injury risk is approximately proportional to the impact forces on a person, which in turn are proportional to the square of the impact speed. These two effects can be summarized in a general rule of thumb: When travel speed increases by 1%, the injury crash rate increases by about 2%, the serious injury crash rate increases by about 3%, and the fatal crash rate increases by about 4% “
http://www.tsc.berkeley.edu/newsletter/winter2008/speed.html

Since almost no one drives at the speed limit or below, and speeding is the single largest risk factor in fatal crashes, clearly people do not consider safety to be their first priority.
If safety outweighed all other concerns, perhaps we should all drive tanks to where ever we are going.
Or, better yet, instead of wasting fuel in the supposed name of safety, just ride a bike.
JacobAziza (author)  JacobAziza6 years ago
Maybe part of your comment is because you are unaware of this point: the brakes and steering still work without the engine. They are just harder. You have to push the pedal harder and turn the wheel (slightly) harder, but the systems are still fully functional.
Agreed. It's easy to coast with my Mazda 323. It's harder in my wifes Tahoe 4x4. But it still CAN be done safely. I do it nearly every day. It just takes a little extra attention and effort.
He probably read all the way to step 4, then his brain turned off, and he skipped the rest
JohnMichael5 years ago
While I am in favor of improved mileage, some of the modifications in this instructable are unsafe and illegal in most places.
Care to back that up with some references? 

Coasting with the car in neutral or clutch engaged has been illegal in my state since 1971 (and is illegal in many others) -http://www.leg.state.vt.us/statutes/fullsection.cfm?Title=23&Chapter=013&Section=01121

So when I am coasting in drive with my high stall torque converter, and the rpms sit at 1200rpms with my foot off the gas, I guess you could say that's illegal, since it's not technically grabbing a gear.

Not at all - if it's in drive you are in compliance. It's coasting in Neutral and/or shifting into Neutral (or for a manual neutral/depressing the clutch) with a vehicle in motion that's at odds with the law.

Now prius drivers can finally get back at Prius Repellers (big trucks) because they can drive one and enjoy the same MPG.

Rportal5 years ago
Driving slowly is illegal. It is called impeading traffic and you can be cited. Just so you know.
JacobAziza (author)  Rportal5 years ago

Driving slowly is not illegal in a single state in the US. The posted speed limit is, without exception, the UPPER LIMIT to how fast you are allowed to drive. It is NOT a mandate that you must drive that speed, it is NOT a recommendation, and it is most certainly NOT the lower limit.

If you deliberately came to a complete stop on the highway, then you are impeding traffic.
If you were, for example, driving 15mph in a 65 zone, you could be cited for a number of things.

However, even then you are not actually impeding traffic, because on a multi-lane highway, people can just change lanes, and on an undivided country road people can pass in the oncoming lane or at turn-outs.
On city streets or mountain roads where it is not safe to pass, it is also not safe to drive 65mph, so there is no problem.

No where do I suggest driving 15mph on the highway. Below around 45 or so (depending on the vehicle) you have to shift into a lower gear, and therefor your mileage drops off.

There are some (not all) states in which there is a minimum speed limit on some highways. When there is IT IS CLEARLY MARKED ON EVERY SPEED LIMIT SIGN.
In Michigan, for example, on some freeways with a 65mph upper limit there is also a 45mph lower limit. This means you can go 20mph slower than surrounding traffic (that's assuming no one is speeding)

Bottom line is, you NEVER have to base your speed on what people around you are doing. If every single person on the highway is speeding, except you, then every single person on the highway is breaking the law, and you aren't.

Don't just make up stuff that you wish was true. Almost all laws are posted on the internet, so you can find and cite the exact vehicle code you want to inform people of, instead of spreading false rumors and confusing people.
actually it is up to police descretion as to what is an impedment of traffic (which you can be charged for) and their working model is different state to state but if you stay within 20% of the limit(depending on conditions) you won't have any problems. Oh by the way you can be charged for driving at the speed limit as well if the conditions are bad. It is called negligent driving (for not adjusting to the change in conditions). Unfortunatly if you are charged with either offence it is difficult to fight in court as I know from personal experience ( I was lucky that I hold a raceing license as well therfore my knowledge and level of expertese was considered better than the police officer's involved )
laws are different in every state. what is illegal in one state might be perfectly legal in another.
Totally not true about posted minimum speed limits. Wisconsin has a 45MPH minimum speed limit on all Interstate Highways (I-94, I-43, etc) and it's not marked anywhere on any highway sign anywhere in the state. It is the same whether you're on a 65mph zone, or a 55mph zone, 45mph is the minimum. Of course, if the MAX limit is lower, then the minimum limit is lower as well, but there's no point in going 35 in a 45mph zone.
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