by Sam Dillinger (comments: 0)

The Truth Behind Your Gas Mileage

gas mileage

It’s a pretty fair statement to say that good gas mileage is something that interests us all. With the national average price for a gallon of regular fuel at $3.52, and premium fuel at $3.89 per gallon, (AAA’s daily fuel gauge report) getting the most out of the gas you put in your car, is more important than ever. Yet it never seems to work out quite the way it should. You read the sticker on the window when you buy the car, or do your homework online for buying a used car, and armed with the info from the EPA, or other authorities on the subject, you buy your car and proceed to drive. It happens slowly at first. You notice that you stop at the gas station more than what you had originally intended, but it doesn’t really intrude on your conscious thoughts very much. Then you gradually begin to notice that you seem to be at the gas station twice a week, and you still have a fairly new car that was supposed to help you save at the pump. So you go through the process of determining what your actual gas mileage is (fill the tank completely, find out how many gallons your tank is, drive until it’s as empty as it can be and still get you to the gas station, see how many miles you've gone, and then divide the number of gallons in your tank by the number of miles driven) and you find out it’s a couple to several miles lower per gallon than what was advertised. So what gives? Why isn't your car getting the 27 mpg on the highway that it advertised, or the 17 mpg in the city? The answer lies in the way the fuel mileage is calculated, and in the way it’s reported.

Let’s start with how the fuel mileage is actually determined. The car manufacturer performs the fuel mileage tests themselves and reports the results to the EPA, then the EPA audits between 15-20% of the vehicles reported. The vehicle gets hooked up to a dynamometer for a series of drive tests. For those of you who don’t know what a dynamometer is, it’s basically 2 very big rollers in the ground that will allow the drive wheels of the vehicle to turn without the car actually going anywhere, as seen in the picture above. It’s used to calculate horse power as well as fuel mileage, emissions, and other tests. The vehicle is chained down securely to make sure it can’t take off. This is a direct quote from fueleconomy.gov “On the dynamometer, a professional driver runs the vehicle through a standardized driving routine, or schedule, which simulates “typical” trips in the city or on the highway.” So the vehicle never gets driven on the road to see what the actual fuel mileage is. It’s driven in a laboratory on a machine and it’s put through a series of tests. There is a hose on the exhaust which collects all of the emissions. The amount of carbon in the exhaust indicates how much fuel was burned during the test, which reflects how much fuel would be used.

Tests such as this one: The A\C test. This test is run with the A\C on the whole time, and with the temperature in the laboratory at 95 degrees fahrenheit, to simulate summer driving. The duration of this test is 596 seconds, or 9.93 minutes. The top speed reached is 54.8 MPH, the vehicle is in max acceleration for 5.1 seconds, there are 5 complete stops during the test, and the average speed is 21.2 MPH. How many of you drive a car in this fashion? I thought as much, neither do I. Again, the test is designed to simulate a “typical” drive cycle. So the problem becomes that the test is an estimate, not an actual standard. This is 1 of 5 total tests that are run, all under very specific conditions. The test doesn’t account for wind resistance either, as the vehicle is not actually moving anywhere. Those of you who live in a really humid climate (like I do) can attest to just how much the air can weigh on really hot days. The EPA is actually pretty candid about the fact that it’s an estimate, but the car manufacturers are somewhat less forthcoming with that information.

There is also some discrepancy on which vehicles are tested. Most people assume that since there is a sticker on the window when they buy the car, that has the fuel mileage listed on there, it must’ve been tested. Unfortunately, that’s not the case. It isn’t even any of the cars on the lot that you happen to be shopping at. Here’s another direct quote from fueleconomy.gov “Manufacturers do not test every new vehicle offered for sale. They are only required to test one representative vehicle—typically a preproduction prototype—for each combination of loaded vehicle weight class, transmission class, and basic engine” In other words, they test one each of a car, truck, or SUV, per manufacturer, and those test numbers are used as the results for ALL of those vehicles that get sold in the country. To top it off, it’s not even a vehicle that’s going to be sold, it’s a prototype. Prototype vehicles have the highest quality parts for the drivetrain (engine, transmission, differential, etc) and the rest of the vehicle. This will give them better results on the test as well. The prototype vehicle changes when it goes to production for a number of reasons, cost of production being chief among them. The manufacturer has to cut down on the cost as much as possible to make the vehicle both affordable for you, and profitable for them.

With all of this information now in hand, what can you do to make your fuel mileage a little closer to the advertised MPG? There are a few things you can do to help maintain a decent average.

  1. Tire Pressure- Your tire pressure is extremely important. The lower the pressure in your tires gets, the harder it is for the engine to move the vehicle. It can cost you a lot more than you realize. Per the EPA, properly inflated tires can improve fuel efficiency by 3.3%, translating to 8-9 cents a gallon at the gas station.
  2. Driving habits- This one is a can’t be stressed enough. If you're always flooring the accelerator to get from one stop light to the next, and riding the brakes hard to stop in time, this will have a huge effect on your fuel mileage.
  3. Vehicle maintenance- If all of the mechanical systems of your vehicle are operating correctly, this is the surest way to get the best gas mileage that you possibly can. Things like- regular tune ups, replacing the fuel filter once a year (if your car has one that can be serviced), regular oil changes with the correct weight of oil, semi annual vehicle alignments to keep the tires from wearing irregularly, and several other maintenance items. This does not mean you need to have the brake fluid flushed!
  4. Vehicle repair- If you feel like your car is running sluggish, doesn’t accelerate as good as it used to, if it’s revving very high while you're driving, if the check engine light comes on (for more on check engine lights visit www.mycheckenginelight.com) if it chugs, stutters, hiccups, or any of these things it’s trying to tell you something is wrong. The longer you wait to have it addressed, the worse your fuel mileage will get.

So when you or someone you know is car shopping, and you see all the commercials shouting at you about the fantastic gas mileage that this particular car will get for you, remember to take that with a grain of salt. Someone, somewhere, under the exact perfect conditions might be getting that advertised gas mileage, the rest of us will have to learn what it’s going to be for us, and how to enhance it as best we can.

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Meet Sam Dillinger

My name is Sam Dillinger. I've been a professional, dealership technician for 18 and a half years. My first introduction to mechanical repair was when my own vehicle broke down in the fall of 1995. I was 18 and couldn't afford to pay to have it fixed. So I borrowed tools and asked a ton of questions and, eventually, was able to replace the clutch on my truck by myself. During the course of that project, I found that I really enjoyed having a wrench in my hand and figuring out the puzzle of disassembly and reassembly of a motor vehicle.

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