Most everyone knows about fuel economy but not many of us understand why our cars get the mileage they get, or why cars get worse mileage in the city than they do on the highway. Hopefully I can help take away the mystery with this article. Also, to determine if a supposed fuel saving idea is spam or legitimate, you need to understand what drives fuel economy in a gas or diesel car.
Your car is a set of systems that work together, and each system has an impact on fuel economy. In this article I’ll go over each system and show you the impact they have on your car’s fuel economy. Then we can test fuel saving ideas against these systems to see if they are likely legitimate or a gimmick.
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Step 1: Your Engine
72% – 78% of the energy that goes into a gasoline engine never reaches the driveline. The energy is lost through:
Cooling (the radiator) and Exhaust heat (58% – 62%) – Gasoline engine temperatures must be kept in a narrow range for the engine to run well, to produce minimum emissions and to reduce wear. And to maintain this narrow temperature all excess heat is dumped out through the radiator. Every bit of the heat leaving through the exhaust is wasted energy. These two heat losses together represent over 2/3 of the energy lost by your gasoline engine.
Parasitic Loss (4% - 6%) – Everything attached to your engine including your water pump, alternator, etc.
Pumping (~4%) – This is the energy needed to draw air & fuel into your engine and to push the spent fuel out of your engine and into your exhaust.
Combustion (~3%) – The energy needed to burn your gas.
Friction (~3%) – Everything that moves or slides across something else experiences friction and the same is true with all of the working parts inside your engine. Your motor oil works to reduce friction but it does not totally eliminate friction. Your bearings add friction, your valve train components add friction, and so do your engines pistons as they move up and down in the cylinders.
Just like a gasoline engine, most of the energy that goes into a diesel engine never reaches the drive-line. But a diesel has an advantage over gas even before it’s burned. One gallon of diesel fuel contains about 11% more energy than the same gallon of gasoline. This means that gallon for gallon, diesel fuel can work 11% harder than gasoline, so everything else being identical a diesel engine should be about 11% more efficient than an equivalent gasoline engine.
But there are other factors that even improve efficiency more. A diesel engine is more thermally efficient than a comparable gasoline engine. In common terms, more thermally efficient means that for every BTU of fuel burned less energy is lost through the radiator and exhaust. Also, a diesel engine produces more torque at low RPMs than a gasoline engine and torque is what does the real work. Torque is what ultimately turns your car’s drive wheels and pushed your car down the road. Lower RPM torque means the diesel engine produces more power at lower RPMs and this also means a more fuel efficient engine.
Add everything up and a diesel equipped car or truck is usually about 40% - 50% more efficient than an equivalent gasoline engine. So, why aren’t we all driving cars with diesel engines like many Europeans? My personal opinion is GM did such a bad job with their late 70’s to early 80’s Oldsmobile diesel car engine that they damaged the USA market and manufacturers are slow to re-enter the market because they don’t want to get burned again.
Step 2: Non-Engine Factors
9% - 12% of the energy delivered to your engine is lost through wind resistance. This is the energy needed to push your car through the air plus the energy needed to pull air back-in behind your car. The faster you drive the more wind resistance your car experiences and the more horsepower your engine has to develop to overcome that resistance.
5% - 7% of the energy delivered to your engine is lost through rolling resistance. This is the energy required to roll your tires down the road while supporting the weight of your car. Most of this is tire sidewall flex generating heat.
5% - 7% of the energy delivered to your engine is lost through slowing down and stopping. When you apply the brakes all of the energy that is moving your car foreword is transferred to heat. This heat is soaked up by your brake rotors then it’s radiated into the air. Braking is the biggest contributor to the difference in highway and city MPG, the second is changing speeds. When you are on the interstate you rarely brake so you don’t have any braking losses to subtract from your fuel economy.
5% - 6% of the energy delivered to your engine is lost through the drive train. These are the parts that transfer power from your engine to your wheels. The components are your transmission, drive axles and possibly a drive shaft. Almost all of this energy is lost through heat and that heat is caused by friction of all the internal parts.
~3% of the gas or diesel fuel spent waiting in your driveway and at street lights & stop signs is called idle loss.
Step 3: The Driver
All cars manufactured since the 1980’s have published MPG ratings and we all know people who “claim” to get much better than those published ratings. While many of these drivers are outright lying to you or deceiving themselves, there are actually drivers who get much better mileage than what’s published for their cars. For the most part these people are ultra-conservative drivers. They beat the mileage ratings by doing nothing more than taking off slow at traffic lights, slowing down while approaching a light to minimize braking and driving at or below the published speed limits. Some folks call this “Granny driving” but it works!
But there is the opposite too! Launch your car out ahead of traffic, run to a light then clamp down on the brakes, drive over the speed limit, and you should expect lower gas mileage than what’s posted on the sticker!
Step 4: Maintenance
Experts state that under inflated tires can cost you up to 3.3% in efficiency, and the impact can be even worse in very high efficiency vehicles. This is because as your tire pressure drops your tire sidewalls flex more, generating more heat and causing more rolling resistance.
Modern engines run at optimum tune at all times, but parts wear out as you drive your car. Coil packs fail and spark plug gaps widen over time. Also sensors fail and when they do your cars ECU (computer) makes false calculations costing you MPG. The best defense is to have your car serviced based on the manufacturer’s recommended intervals. But it’s just as important to take your car in any time something does not “feel right” or any time your check engine light comes on. This is because regular service will not take care of random sensor or other non-serviced part failures. I can almost guarantee that whatever a necessary service costs you now, the long term cost of not doing the service will be higher in MPG loss plus you will need to have the service done later anyways.
Except for some sports cars, when your car was designed the Engineers chose a wheel alignment that gave your car optimum MPG and optimum tire wear. Just like everything else on your car, suspension components wear and all of the angles that add up to a proper wheel alignment change and these changes impact your MPG as well as higher tire wear.
Other Maintenance Issues
Anything else that creates a new noise while you are driving or braking has the potential of costing you MPG. For example, dragging brakes generate extra heat and this costs you MPG. So will any other new noises that occur when you are driving down the road.
Step 5: Fuel Saving Gimmicks
These are all what I consider gimmicks. Some are outright fraud while others while based on science, are beyond the average driver’s technical abilities. The first test to separate a gimmick from a real fuel savings device is to apply a very simple test – are the claims “too good to be true”?
Gas Tank Additives
It’s been proven over and over again that gas tank additives have zero impact on fuel economy. Next time you are in your local big box or parts store, read a few of the labels. With no exception they all claim increased fuel economy – every one of these fail my “too good to be true” test. But why does your local auto-parts store sell so many? Because they sell!!!!
ECU (Engine Computer) Re-programmers
ECU re-programmers are devices that modify your ECU’s programming. There are after-market “chips” or plug-in modules available for many different trucks and cars on the market. And although most actually do what they claim, the change is always at the expense of something else. You may get more horsepower at the expense of worse mileage or better mileage at the expense of performance. Aftermarket “chips” are good examples of “you can’t get anything for free”. You are trading up at the expense of a down trade somewhere else.
A even worse product are the re-programmers that let you modify your own ECU settings. My issue with these re-programmers is I don’t know more about all of the ECU parameters than the Engineers who wrote and tested the software. Do you? Their software is based on hundreds of tests and I am only working on one engine – mine! I see re-programmers that let you modify your own ECU settings as the perfect opportunity for me to screw up my own engine.
Back in the carburetor days an exhaust modification done by someone who knew what he was doing could easily net 1 – 3 MPG by reducing back pressure, but those days are long gone. With today’s engines, you can modify your exhaust to reduce back pressure but it’s “hit or miss” with a miss more likely than a hit. Most exhaust modifications are done today because they "sound better".
Your modern engine oil is already loaded with all of friction reducing, acid neutralizing, and wear reducing elements it needs and I doubt seriously that adding a bottle of “magic elixir” will improve anything. Next time you are in your local big box or parts store, read a few of the oil additive labels. With no exception they all claim increased fuel economy and/or less wear and every one of these fail my “too good to be true” test. Why does your local auto-parts store sell so many oil additives? It's also because they sell!!!!
There is one exception that I believe in - oil thickeners. If you happen to have an engine that's at the end of its life and it’s burning a lot of oil then adding a bottle or two of one of these will help a little. They work by increasing your oil’s viscosity (thickness) and the thicker oil blows past your rings less. But you are just avoiding the inevitable!
Hydrogen generators are a good example of supposedly getting “something for nothing”. The systems I looked at use battery power to break down water into hydrogen and oxygen then the gas is sucked into the engine and burned. My issues with this one are based on science.
Your modern engine is already highly tuned and running at peak efficiency. It has to be to pass modern EPA regulations. So, how can injecting a little bit of hydrogen and oxygen help?
They use battery power to make hydrogen and where does the power come from? Your alternator! And what drives your alternator? Your engine! So, even if somehow the hydrogen increased your engine’s power, all of that increase plus some would be used to supply the additional load that your alternator needs to generate the hydrogen! There is no such thing as a perpetual motion machine.
Nitrogen in Tires
To me this is a big one. In theory filling your tires with nitrogen will save gas mileage because your tire pressure will vary less, plus pure nitrogen promotes less rim corrosion and less dry rot. My issue with this is if you keep your tires at their proper pressure the only direction your pressure can go is up as your tires warm up and this increases mileage! And anyways your tires already have nitrogen in them. Air is made up of 78% nitrogen, just under 21% oxygen, and the rest is water vapor, CO2 and a few other minor gasses.
One exception I have is with classic cars. I see value in putting nitrogen in classic car tires because removing oxygen from classic car tires help to preserve the rims by reducing corrosion and tire dry rot.
Yes I said it – our own Government sponsored E85 is a scam!!!!
Let me clarify. The issue with E85 is that one gallon of E85 contains 27% less BTUs than one gallon of gasoline and a flex fuel car will get lower mileage on E85 when compared to gasoline, some tests show 26% lower mileage. But E85 will only cost you about 11% less than gasoline. The result is burning E85 instead of gasoline will cost you more per mile than burning gasoline.
But don’t believe me, read this article.
Fuel Line Magnets
We’ve all played with magnets when we were kids. Its fun to see opposite poles attract and like poles push apart. And these fuel line magnet peddlers are depending on these memories when they try and sell you their snake oil. These guys claim to increase your fuel economy by magnetizing or “straightening out” your fuel molecules. What happens when you put a magnet against a glass of water or against your skin? A whole lot of nothing! And guess what happens when you put a couple of magnets against your fuel line? A whole lot of nothing! The reason is except for a MRI that uses a super magnetic field to temporarily polarize the water molecules in your body, magnets have no effect on non-magnetic items like the gas or diesel fuel in your car.
Many years ago I experimented with water injection and it worked – I got one more MPG. But this was in a carbureted V8 engine built in the early 70’s. These days water injection is more likely to upset the tight balance your engine is running. In other words, you are more likely to do more harm than good by water injecting a modern engine.
Any other “Magic Pill”
Any other “fix” that makes extravagant claims is a guaranteed fraud. And if it passes the “too good to be true” test it still may be fraud. You just need to apply the following tests.
Can they prove it reduces your engine’s heat loss?
Can they prove it reduces your engine’s parasitic loss?
Can they prove it reduces your engine’s friction loss?
Can they prove it reduces your engine’s pumping loss?
Can they prove it reduces your car’s wind resistance?
Can they prove it reduces your car’s rolling resistance?
Can they prove it reduces your car’s braking loss?
Can they prove it reduces your car’s drive-train loss?
Can they prove it reduces your car’s idle loss?
If not then it's just another gimmick designed to steal your money!
Step 6: Legitimate Fuel Saving Ideas
There are some proven fuel savings ideas out there and all are based on real science.
Energy Saving Tires
Some of the major tire manufacturers have developed energy saving tires and they accomplished this by reducing the tire's rolling resistance. Although it’s not worth replacing good serviceable tires with energy saving tires, it’s always worth buying energy saving tires when you replace your worn tires.
Something else to consider is if your car came from the factory with energy saving tires you will lose efficiency by mounting a set of non- energy saving tires.
Roll Up Your Windows
Most modern cars are more economical above 45 MPH with the windows rolled up, even with the A/C on. This is because rolling the windows down increases the car’s wind resistance and the increased wind resistance has a direct impact on fuel economy. But a lot of drivers still believe they are saving money by turning their A/C off and they roll their windows down.
Car Body Modifications
The body of your car is a compromise of form, function and esthetics. Modifications can be made to reduce your car’s wind resistance which has a direct impact on your fuel economy. Check-out http://ecomodder.com/ to see some extreme car body modifications in action.
Reduce Your Rolling Weight
How much un-needed weight is riding in your trunk or back seat? Believe it or not even a few hundred pounds of “stuff” will have a slight impact on your mileage. So, if you don’t need it, take it out!
Drafting is extremely dangerous but it works!!! Most have watched NASCAR drafting, where a following car pulls up close to a lead car. Drafting works by reducing the drag resistance of the lead car and reducing the wind resistance of the following car - both cars win. And the same would be true between two cars on the interstate. The problem is if you are the following car your reaction time can’t be fast enough to react to changes the lead car is making. In other words, don’t try this!!!!!!!
If you do decide to try drafting then staying a few car lengths behind a semi-truck will make a difference and riding between two trucks in the same lane will make a greater difference. Just remember that if you can’t see his mirrors then he can’t see you! Draft at your own risk!
Conservative driving is by far the greatest impact you can have on your fuel economy and here are a few tips that help.
Give yourself 2 seconds between your car and the car ahead of you. If the car in front of you brakes, you can coast for a while without hitting your brake. Sometimes this is impossible in aggressive heavy traffic because other cars will swing into the space in front of you.
If you see a red light or a “stale green,” a long green light that might go yellow, take your foot off the gas and coast to the light. The idea is to use your brakes as little as possible. Sometimes you can coast a while then brake for the light and sometimes you can coast until the light is green. Every bit of that coasting is “free miles”.
Accelerate slowly from stop. Fast acceleration takes more horsepower and more horsepower = more gas burned.
Drive slower. The experts say that every 10 MPH above 65 MPH reduces your mileage by 10% - 15% and I believe it.
Run the correct octane fuel. Many higher performance cars require mid-grade or premium fuel. And while these cars will run fine on lower octane fuel, they are able to do so only because the engine detunes itself. The problem is a detuned engine is less efficient than an engine running at peak performance and more often than not the cheaper per gallon cost is offset by lower MPG.
Buying a more fuel efficient car will definitely save fuel, but be careful if you plan to trade a older less efficient car that's in good shape for a new more fuel efficient car. Often you will come out on the short end because the increased efficiency does not cover the cost of your new car!