Step 9: Steps 19 & 20 - Major Modifications

Cold Air Intake/High Performance Air Filter

A better air filter and intake make it easier for your engine to breath, and can increase you
Mileage another 5% or so. It really depends on the car; some cars have very free flowing intakes, some have an extensive system of baffles and poor air filters. Replacement free flowing air Filters can cost as little as $20 online and are defiantly worth the money for most.

Free Flowing Exhaust

Free flowing exhaust is a change that, while it can increase mileage, is usually more trouble
than it's worth. If your engine is breathing in more easily, you also want it to breathe out more
easily. The trouble is, exhaust systems cost hundreds of dollars plus the installation costs, and unless chosen wisely can hurt mileage as much as they can help.

Decrease Drag

If your car has a roof rack you don't use, or a radio antenna you could do without, or a rear hood ordainment you don't particularly car about, get rid of them. If you want to get a little more serious, you can use foam, Plexiglas, tape or the like to smooth over places on your car that are not aerodynamic. If you own a truck, a truck bed cover or a rear cab can also very much increase your mileage. On some cars (research this first) a rear spoiler can help aerodynamics, though commercially sold spoilers are not designed to do this.

If you really want to go crazy, you can follow this guy's lead, and get 100 miles per gallon.
it took a round trip from Chicago, Illinois to Canton,&nbsp;Ohio to get my wife to believe this one. She drives a Honda Civic Si and it says PREMIUM&nbsp;FUEL&nbsp;ONLY on the door for the gas cap. i told her to fill up with premium for the trip there and then run the tank almost empty while there and then only use regular on the way back. She took my ScanGaugeII and got 31.7 MPG&nbsp; on the way there and 30.9 MPG on the way back. And for all we know that variation could have been caused by wind. Either way that .8&nbsp;MPG increase isn't worth the extra 20-25 cents a gallon.<br />
That's really not a valid check due to altitude changes, wind speeds, ect, different brands/mixes, ect. Chicago is one of the cities that uses its own boutique fuel blend, which your Civic may not have liked.<br><br>A valid check would be a 5 tank average under what is your normal driving conditions. Run 87 for 5 tanks, then run 93 for 5 tanks, and see what the average is. When comparing things like this, keep as much the same as possible.
Thanks for the live data, I had often wondered about this. Does fuel choice affect warranty on cars specifying premium fuel?
"50% throttle at 1200 rpm's uses less gas than 10% throttle at 2500 rpm's" Another in an endless line of impossible statistics. Where are the numbers from? They do look impressive and make a person feel good, though. "The engine is more efficient when it does not have to pump air past a closed throttle." An engine does not pump air past a throttle, it sucks air past it. It has no impact on efficiency. Cam profile, injector pulse width, throttle position, and engine RPM dictate efficiency. A Honda Civic I used to own turned 2250 RPMs at 55 to 60 MPH. Honda engineers probably had a reason to have the car in that powerband at highway speeds. I have commented on other Instructables on increased gas mileage, and I will again say please pardon my intolerant tone, but efficiency vs. RPM varies from car to car, and oversimplifying it is misleading. Unsubstantiated statistics and numbers imply that whoever states them knows what they're talking about, further confusing the mileage issue. Lighten the car, easy on the gas pedal.
Make a database on your PC and implement date, mileage, fuel quantity you read on display and calculated on price end expence , fuel price, air filter and oil changes, tire pressure checkings, whatever you consider important and in a few months you'll see what really happens with your car. I do that since 2005, quite boring but you'll find out many actual facts on this subject.
I would argue pumping air past a restriction is effectively the same as sucking it through. That being said, after looking into it further, this only makes a difference at near the engines limits, so it probably isn't relevant. Its the same argument as clean vs dirty air filters actually. And, you did actually catch another mistake for me, I meant to make it clear that the 1200 vs 2500 was specificly the results for a vehicle I own. You're absolutely right about honda engineers having a reason the engine maintains that speed, and that every car is different. Thats the whole point of this page of the article. In your case, the "sweet spot" that Im recommending you try to stay around or below would be 2250. What I found is that engineers usually design their cars to be most efficient at highway speeds (no surprise), and that by operating your engine at a similar rpm as you would on the highway, mileage can increase.
Also, for a little more lovely math.<br/><br/>Im just pulling off a generic Toyota fuel map. (back calculated volumetric efficiency based on MAP readings, engine load, throttle position, ect)<br/><br/>VE at 1000rpm, 50% throttle = 71%<br/>VE at 2500rpm, 10% throttle= 54%<br/><br/>to find the relative amount of fuel being injected (assuming all other factors equal), simply multiply rpm by VE.<br/><br/>710 at 1000rpm 50% throttle, 1350 at 2500rpm, 10% throttle. <br/><br/>710/1350=~53%, <br/><br/>meaning 1000rpm 50% throttle uses around half as much gas as 2500rpm at 10% throttle.<br/><br/>This is assuming certain other variables are constant, which is likely not the case, so this figure is an estimated example only.<br/>
Your work to support your claims surprises me. It's no secret that the tone in my citicisms was very dismissive. We now fing that you are not simply shooting from the hip, but instead have made very informed statements about mileage. Good job.
You were quite right to criticize, I provided none of the reasoning or math for my suggestions in the article, and with the unfounded claims that get thrown around on the internet, I should lend credence to myself and show my work. To an extent, it's still shooting from the hip, I cant say these figures are always true for every car, but they are tips and guidelines, not a scientific paper full of hard-fast rules. My inclination was that the reasoning and math would be beyond the scope of the article, and beyond what most people would care to read, but perhaps the average instructable reader wants a bit more.
Any car buil in the last 20 years or so has a sealed fuel system, parking in the hot sun or in the shade makes no difference at all since any fuel vapors caused from evaporation are captured inside the sealed system and will return to a liquid state once they cool down. In fact, newer vehicles ("96 and up) generally have a sensor built into them so that if you don't tighten your fuel cap all the way (to capture the vapors), your check engine light comes on. A good idea if you drive an older car I guess, but useless for anything "recent".
That's true, there are two pipes from the fuel tank, one comes from the fuel pump inside the tank, the other takes fuel back from engine to the fuel tank. Consider that the fuel pump cools and so endures more it's working life if submerged by fuel. That means more or less something like two gallons fuel in your tankat least. Remember it does exist a fuel filter too between the fuel pump end the engine, better to clean , possible in some diesel engine, or change. It surely helps pump life too and I can tell gas pump failure is a real bad nightmare, I changhed mine in a parking lot, a most difficult task I ever tried . Gas filter in my car was almost filled of an unclassified black dirt after 50.000 miles when I changed it . <br>Clean new engine air filter and a well checked tire pressure are mandatory as it results in my personal mileage, fuel, oil and filter changes database. Keep a hand or pedal air pump in your trunk it may help or even save a lot.
so why do new cars have a fresh air tube on them?? If the system is sealed and you pump all the gas out of the tank you'll have a vacuum in the tank and won't be able to get the cap off to refill it.<br />
im having a hard time figuring out where you are getting your data from
Depends on which data you mean. A lot of it is personal data collection using the vehicles I have access to and an OBDII laptop connection and log tool. Other parts are government sources, as well as other sources.
i guess it would be the one on your speed reflecting fuel mileage, i know you lose fuel economy the faster you go but 3 mpg per 5 mph? that seems a little drastic to me. i drive a 2005 f150 with a 5.4l, at 110 kmph (i live in the great white north) i pull 21 mpg over a distance of 450 kilometers and use roughly half a tank, now at 145 kmph i get slightly less then that, i think the last i checked it was roughly 19 mpg. sorry i have no idea where i was going with this, i sorta lost my train of thought half way through.
Good hints!. Still, a very important one has been missed (i guess): get a low mpg car. Imported Europeans are the best/more common on fuel saving. <br> <br>For instance, I have a little Citroen (diesel, 5 seats, 1992) in which I ride every day to university. Most is country roads, but include some city traffic and a few pedal-to-the-metal moments. I never went below 52 mpg (us gallons). Back in the 90s, an advertised announced a best of 64 mpg for this car. It held a record for economy until 2008, lost for a Peugeot HDI) <br> <br>Cars that go below 25mpg are quite rare in Europe. They're either big luxury cars, US cars, or pre80's. Maybe because we rely on small blocks, I don't know....
if your going down a hill, throwing your car into neutral is actually detrimental to you fuel economy. Most new cars (2001 and up) are fuel injected, and when coasting down a hill in neutral, your car uses x- amount of gas to keep the engine idling. If your car is in gear going down a hill, the injectors will supply the engine with less amount of fuel than it would while idling. think of it this way...if your coasting down a hill under no acceleration from fuel, then the tires are spinning the engine, so why should the gas have to?
I also meant to add, keeping your car in gear also slows your car down depending on your speed, gearing, incline, ect. This might be fine, but it might also be a waste of potential energy you could be using to speed your car up. Cruising a little faster down hill means having a little more momentum to go up the next. (staying within safe speed limits of course)
This is true on some fuel injected cars, but not others. For example, I tested this in a 2009 Mini-Cooper about a week ago. According to it's ECU hooked to my ODBII reader, it used several times as much gas when decelerating in-gear than it did when out of gear, depending on the engine RPM. I've tried it on other cars as well, with abut 50/50 results. <br><br>
That was an ok way to save on gass but we need more modifcasons&nbsp;than driveing habits.
the problem is there are only so many mods that will help, I'll be doing an instructable later this summer on mods I'm doing to my jeep. I currently get around 26 MPG on the highway in my '05 Jeep Liberty and the EPA&nbsp;tested it at 21 MPG&nbsp;on the Highway (an almost 20% increase). And that was mostly from changing my driving habits.<br />
Anyone remember when the national speed limit was 55mph? Seems to me I recall it had something to do with energy conservation. Well, flash forward to today. I have been advised by a very experienced mechanical engineer to slow down to that speed on the highway to get optimal gas mileage. Today I tried it. My gas mileage on my late model Jeep Cherokee while running 75-80mph on the interstate is a very disappointing 19.5. When I slow down to 55mph on the return trip today, it went up to 27.1. And driving more slowly decreased my stress. I noticed a lot of other drivers have slowed down, presumably since gas has increased to the current prices. I'm thinking that perhaps there would be less stress on the roads, and perhaps less deadly accidents, if we all slowed down. I know I'll be happily over in the right lane poking along from now on.
Wait, FSJ Cherokee, xj cherokee, or liberty (Only a cherokee in non US regions)? For the first two: HOLY CRAP! Nice dude! For the last, isn't it rated for 27 stock?
i know this is a little late. i have an '05 Liberty and get around 26 on the highway, it's rated 17 city/21 highway, that's almost a 20% increase. i have also tweaked&nbsp; it a little bit(the car not the engine), but before those changes i was getting around 24 about a 14% increase.<br />
Thats absolutely true. Most cars are designed to be the most efficient at 55 mph. Some a little less, some a little more. (My car seems to do the best at 50, our van seems to do the best at 65). Everyone should try to figure where there peak mileage is, and be there as much as possible. For many, that just means slowing down a little :)
your nighttime fill up maths is wrong... 1.16% of $80 is $0.928 so you still save a buck, but certainly not $10 :)
Oh lord, thanks a lot. ill fix that straight away.
hehe no worries :) i think your point is still valid, especially for people who use a lot of fuel... a couple of bucks a week adds up over the years!
But if you look at the pump next time you fill up, you'll see the sticker says the amounts are temperature adjusted (at least the pumps in Ontario Canada are).
One station I use (Costco)&nbsp;does not temperature compensate, most of the others do. <br /><br />You can always ask the attendant what the tank temp is, it is in a report they generate every time the tank is filled. <br /><br />The tank temperature will change little over the course of a month, except when it is filled. In the winter, fuel after a fresh delivery, in the summer, fuel before. (If you know the patterns.)<br /><br />The number 1 fuel economy fix I have found is to get a real time fuel economy meter. (Scangauge for newer cars, MPGuino for older fuel injected types.) You can instantly see what works and what doesn't.<br />
Its not temperature adjusted in Maryland where I live, but thats good to know.
Correction: Pumps are now temperature adjusted.
"Driving around looking for a better spot...uses a ton of gas" A ton. Wow. How about very, very little. You're driving me nuts! "Keeping your tires properly inflated, your oil changed, your transmission serviced..." OK, good. Sound advice. "...your air filter clean, fuel injectors cleaned..." Oh no, you were so close to getting through a whole step without repeating a falsehood! An air filter would have to be literally blocked off to have an effect on mileage. Why? As a filter becomes more restrictive, the engine knows nothing more than the total flow though the MAS, flowing the right amount of gas for engine RPM, load, throttle position, etc. Less flow through the filter causes you to push the gas pedal further, sucking more air, but having no effect on the engine managements system's calculations, again all based on flow through the MAS. "Any one of those factors if left out of check can hurt mileage by 10% or more." Another statistic pulled out of tin air. I'm sure you believe it, but whoever came up with it has an agenda.
Wow, just picking apart the article... Okay. Uses a ton of gas: Define a ton? To me, if driving around the parking lot for 5 minutes uses enough gas to get you a couple miles down the road, you're using a ton of gas. Moving around a parking lot, at least in my experience, is nothing but speeding up a little, then almost immediately slowing down, repeated over and over again. As for fuel injectors, you're suggesting a clogged fuel injector will produce the same fuel atomization, power out, and combustion efficiency as a new one? Does that make any sense to you? And as far as air filters go, I can't really do anything but flat out disagree with you there. You are correct, the ecu will maintain proper air fuel mixture, but thats not the point. A dirty air filter does make a sizable difference in economy. I might give you some slack here, if I hadn't physically observed lower fuel consumption rates on vehicles after an air filter change. Guess what that observed change was on the 2nd to last vehicle air filter I changed? Thats right, 10%. I can see why you'd say you say that, seeing as though youre backed by the government fuel economy website. (They do admit that carbureted engines show a difference, but in their cited article they did not measure differences in FI cars) If you havent already, take a look at the article. You clearly have a good eye for picking out problems, you'll find plenty in that one.
I do have a good eye for picking out problems, to the point that I come accross as arrogant or argumentative, this being a prime example. Not my intention; I have simply seen too many unsubstantiated claims. You have shown that you are magnetudes past the average "I read somewhere" guy. I myself have never observed a significant increase from an air filter change. My observations could be affected by- air filters may have not needed replacing, the cars were high performance and had filters with perhaps 100% more surface area than required, or any number of variables logging MPG before and after the filter change. As for the fuel injectors, atomization and pattern will affect power output, but an injector simply running at lower capacity due to a restriction will be corrected by the ECU with a longer pulse width or duty cycle, depending on the system. Let's call it a draw. With the edge to you. Just a slight edge.
Lower output capacity isn't a particularly a concern. If you ever have the occasion to use a fuel injector outside of a car, try using one that's fouled. A normal injector puts out a nice, even spray. A plugged injector does the same, only with fewer jets of mist. A fouled injector (Im making up this term, Im assuming injectors in this state either have a carbon film on them, or the individual ports are partially plugged) will spray in odd ways. One of the ports might spray a straight, non-misting stream, one might be spraying at an extreme angle, one might be spraying intermittently. I only comment again to say its interesting to see injectors that arent "plugged". Plugged injectors probably dont cause as much of a problem. Id also imagine (guessing) that fouled injectors would be easier to clean, as the solvents can penetrate the individual orifices more easily. Also i should concede Im probably misinterpreting some of my past experiences. Previous to the last air filter change I did (which was just an air filter), I replaced an entire intake system on my brothers car. He's averaged about 3 miles per gallon more (~15% increase) and Ive been attributing that to the air filter, though it could just as easily, and probably more likely be the temperature of the air, which is now coming from a different place in the engine bay.
Yep, changing more than one component can muddy up what really works and really doesn't. Drag racing thing, change one thing at a time when going for that last ounce of performance. Sweeping changes (intake manifold, headers, exhaust) are less important to quantify individually. I would expect a contribution to mileage and power be made by both the filter and cold-air intake in your situatuion. Who knows what contributed to what and how much. I have seen a fuel injector operate in free space when installing a water injection system on my Grand National. It was, of course, unfouled, so I cannot comment. But it looked neat.
It continues... pure nonsense, but backed up by math, so it must be true, right? The temperature past just a few inches underground fluctuates very little. By your thinking, we in MN must be saving an absolute fortune in January. The problem here is, you put this myth right after a suggestion to look up cheaper gas prices. This is an undisputable and valuable way to save a buck in the car. Most people won't look at the two different topics and dispute this one because the don't belive the one about the weight of gas, they will instead believe what you say about the effects on gas at noon because the first suggestion is a truth. Please, hypermilers, environmentalist crusaders, modern-day hippies, stop with the unsubstantiated claims.
I had no idea I was subconsciously organizing my tips to toy with the minds the readers! And by that, I mean the order was entirely random. Actually, this is a claim I have been meaning to take off of here, but not for that reason. Modern gas stations that most of us fill up at (and that most states require) have temperature corrected flow rates. I was basing this on the only gasoline pump Ive had the pleasure of disassembling, that was made sometime in the 40's. Either way, thanks for the comment.
An easly repeated mistake. We all have seen something or another repeated endlessly accross the web, and this was one. But, gone forever now! Perhaps the new number 11 could have something to do with the importance of a properly-functioning emmissions system. Dozens of different parts, each able to cause lost mileage when clogged/broken/even missing.
Thats a very good idea, though its really beyond what I imagine the general readership of this article could do on their own. Overall, Id say that would be lumped with "maintain your vehicle". Maintaining you vehicle with best gas mileage in mind could be an article onto itself.
"Looking at the tech-specs for some of my own vehicles, even under a relatively small engine load, I would have to have my engine out of gear for 12 minutes to make up for 1 minute of acceleration." OMG. Where could you have possibly come up with that ratio? Accelerating at what rate? Up to what RPM? At what road speed? Say it again, folks... lighten the car, stay out of the throttle, the rest is pure amature conjecture and outright untruths.
That's actually kind of a mistype on my part. Let me try to make this a little clearer so you understand. Taking your car out of gear in a manual car means your engine needs to use a little gas to continue to run at idle. So, take the amount of gas that your engine needs to run at idle for 12 minutes. Now, accelerating to make up for speed you could have gained going down a hill, under a small engine load (empty vehicle minus myself, on flat ground, accelerating as efficiently as possible ) to the same speed I would have had going down the same hill, takes roughly 12 times the gallons per minute that popping the engine out of gear does. Most people, driving less conservatively, would have used even more gas. How did I measure this? I logged data using a OBDII scanner monitoring my vehicles fuel consumption. How did I know I was accelerating as efficiently as possible? Practice, being a good and conservative driver, and using the scanner to keep track of overall fuel usage as well as instantaneous usage. How do I know this is decent data to make a claim on? Repeating the test, 3 times, at 4 different locations. Granted, this is different for other cars (as I clearly stated) but its a point to be considered. I do have an idea of what I'm talking about, and I wouldn't call myself an amateur based on my background. Thanks for the comment though. If you're thinking that, it means 100 other people were and just weren't saying so. Glad I could respond and thanks for reading.
I stand corrected. In fact, that's exactly what I asked for. Yes, it is valid for only your vehicle, but logical conclusions can be drawn regarding other cars. A scanner can be a valuable tool. I use mine the other way, though, to squeeze out every last drop of power (which can also increase mileage in some cases).
&quot;Many cars now have an Economy/Sport setting switch, or an &quot;Xtra PWR&quot; button somewhere. These can drop you mileage 50% in some cases.&quot;<br/><br/>*sigh* 50%. In what cases? Another ridiculous statistic. The &quot;power&quot; switch seen in some cars only forces the automatic transmission to shift earlier than it normally would. In &quot;power&quot; mode, it does nothing, in &quot;economy&quot; mode, it short shifts. Using light throttle in either setting will have the same effect, slightly increased gas mileage. Seriously, 50%! <br/>
The "power" switch seen in some cars only forces the automatic transmission to shift earlier than it normally would. True, in some cars this is the case. In others, it completely changes the fuel map the ECU references, ignition timing, and/or valve timing. Yes, 50% is a maximum, and its based on people driving incredibly stupidly. The particular car I tested that on, a volvo 850 turbo, used 50% more gas on a few acceleration runs when in sport mode. I imagine mostly due to shift points, I dont know, I havent checked on that particular car (I think it might also effect boost pressure, some Saabs do this.). That obviously will not transfer into 50% overall gains. Why did I include this number then? because I know in at least one circumstance this is the case, and it gets the point across. Should you expect 50% gas mileage gains? No. Could you achieve the same results by staying off the throttle? Maybe. Those switches not only raise how long you can stay in gear, but some raise the minimum rpm level to upshift. In such cases, When it comes down to it though, cars are a lot better at remembering to drive efficiently than people are, and they're a lot less likely to get tempted to rev the engine up far higher than it needs to go. Either way, fair enough complaint on the stats, I should take that down. Dont think Im just pulling these numbers out of a hat though.
I have read that there is some merit to when to gas up. Buy the gas in the early morning, after the fuel in the ground tank has cooled as much as it will, not in the evening, when the heat of the day percolates down to the storage tank.
Thats a great point! Thanks!
Although I read this article with much curiousity, I'd disagree with shifting to neutral while rolling free or going downhill. Since long time cars do have a fuel cutout relay, which works in carburated as in fuel injected engines, simply cutting off the fuel if the engine is turning and there is no risk of stalling (so over let's say about 1200 - 1500 rpm).... soooo car does not use ANY fuel than! If rolling on neutral, running engine uses fuel for iddling. If you don't believe, ask the expert or watch an adequate "Top-Gear"... or even better, if you have a manual transmission, just try turning off the ignition during rolling on gear. Will you hear ANY change of the engine sound? No! (if you turn down your stereo of course), that's because what you accually hear is the engine turning rather than the explosions in combustion chambers. Than turn back the ignition (but don't push to the starter) - again, not a tiny change in car's sound, although now the engine is ready to take control and inject fuel (open cutout valve) if the rpm will drop below some level. So I would say - roll on gear if you can, besides it's safer on the more steepy hills and safes your braking pads too.
Well, I completely disagree with the concept of a carburator cutting off fuel to the engine when downshifting, you are entirely right. Modern manual transmissions do cut off fuel to the engine when costing down hill. There are even a few automatics (certain CVT equipped cars) that do as well. That does save gas But, keeping the car in gear slows you down. Putting it into neutral lets you pick up speed that you would otherwise have to use gas to obtain, and keep up speed that you would have to use gas to keep. The amount of gas you'd save by turning off gas fuel is far less than what you save by keeping up your momentum. Obviously though, its a case by case basis. Just play around with it, and do what works best for you.

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