Gas Mileage Calculations: Why MPG Numbers Really Matter

Gas mileage calculations have recently changed to more accurately reflect real-world mpg. Here’s what you need to know about the new math and how to improve your mpg, save money and reduce air pollution.

| August/September 2007

We drive much more than 55: Today’s speed limits are much higher than those of 1985, when the EPA last revised its mpg math.

We drive much more than 55: Today’s speed limits are much higher than those of 1985, when the EPA last revised its mpg math.

Photo by Istockphoto/Harris Shiffman

Learn about recent gas mileage calculations and how you can improve your mpg, save money and reduce air pollution.

Gas Mileage Calculations: Why MPG Numbers Really Matter

When Berkeley, Calif., resident Bradley Berman drives his 2006 Toyota Prius around town, the fuel economy indicator pushes into the low 40s. That’s a far cry from the number advertised for the gasoline-electric hybrid, so Berman’s disappointment is understandable.

He says, “When you see 60 mpg on the sticker for city driving, and you’re basically doing city driving but getting in the low-40s, you think ‘That’s terrible!’” Berman’s numbers improve on longer trips, but they still come up short compared to the official miles per gallon estimate for the Prius. Berman should know. As the founder and editor of, he hears from numerous hybrid drivers across the country who share similar experiences.

For better or worse, hybrids have become the poster child for vehicles not living up to fuel economy expectations. That may be because hybrid drivers are theoretically more interested in saving oil and thus complain louder; because hybrids display real-time mpg results and thus their drivers are more aware of the results; or perhaps simply because, as the reigning mpg champ, they’re an easy target.

The truth, however, is that advertised fuel economies of all vehicles, including conventional models, have been slowly but steadily drifting from reality for more than two decades.

To remedy these inaccuracies, the U.S. Environmental Protection Agency (EPA) recently changed the way it calculates fuel economy estimates. The new math will first affect the numbers for model year 2008 vehicles. The results will be lower, more realistic city and highway mpg estimates that “help us capture the way Americans are driving now,” says EPA spokesperson John Millett.

8/10/2009 4:42:49 PM

We can all drive more fuel efficient cars starting today without spending a dime – it’s a matter of adjusting our driving habits. I learned a ton at an eco-driving workshop at this summer’s Midwest Renewable Energy Fair. Check out the top ten tips at Happy eco driving Denise Thornton

9/23/2007 8:49:10 PM

"There is no insult like the truth" Hybrids are the latest crop of Americans wanting absolutely everything for absolutely nothing. The Geo Metro got better real world millage than than any of the current crop of hybrids. It did so with 1/2 the moving parts and 1/4 the price. But Americans don't want sensible cars that get fantastic gas millage with reasonable compromise. Americans want rolling lifestyles with all the gewgaws of home. AND fantastic millage. AND a good price. AND AND AND. No amount of technology can give you something for nothing. Responsible living requires sacrifice and discipline.

9/23/2007 7:20:38 PM

tc is absolutely correct. Come on - making flagrant comments like that (without supporting facts) leads to misinformation and people ranting about non issues. Too much of that already happens. BTW: I have been very annoyed with my 1999 VW Passat and the fact that its impossible to get within 20% of its rated mileage, regardless of driving habits. However, the Passat is quite a lemon and not does not necessarily serve as a good indicator.

9/7/2007 4:41:58 PM

Also, "fill up in the morning" is a complete crock too. The station's tanks are underground at a constant temperature anywhere from 55 degrees to ambient, and don't vary during the day. If it were true, the 'imagined' difference in density of 10 or 20 degrees F wouldn't make a difference anyway it's to small. Again FACT CHECKS PLEASE!

9/7/2007 4:35:29 PM

"But don’t top off — that “extra” gas probably gets sucked back into the station’s tanks." Huh?! That's ridiculous, there's no way gas is going to jump from your tank, up the filler spout, thru a oneway anti siphon valve, and into the filler spout. How about someone doing some fact checks!!!!! BTW, My 04' Corolla rated at 31/41 actually gets 22/33 depending on the tank with a lifetime 30,000 mile avg of 27mpg.

8/9/2007 12:02:55 PM

Come on -- are you trying to give hybrids a bad rap? Anyone that is getting only 40 mph out of a Prius is clearly driving really aggressively or worse. I have owned a Honda Civic Hybrid for 3 years now and am regularly getting over 50 mpg. from this car. How do I do this? By simply driving the posted speed limits. I go 65 on the interstates -- 55 when signs tell me that is what I should do -- 35 when the exit signs say to do that, etc. I live in very hilly terrain and that costs me some in my daily 21 mile commute to work. One direction is more downhill than the other, so going to work I have been averaging 54.8 mpg. and on the roundtrip, 48.4 mpg (and that is with air conditioning running.) On flat terrain, getting over 50 mpg is not at all unusual. Every hybrid car is equipped with instruments in the dash that tell you what your overall mileage is, as well as your instantaneous mileage, so all you have to do is modify your driving habits to maximize your hybrid car's efficiency and that is precisely what all of the hybrid owners I know are doing. I have heard that a friend of a friend has been able to get over 80 mpg from his Honda Inspire (a 2 seater hybrid that is no longer in production.) Since I started obeying speed limits, I am now noticing that I am a distinct minority in doing so -- I get passed by all kinds of vehicles, including semis. In addition to doing this, I now do the following which I can directly see to be improving my car's mileage performance -- use cruise control as much as possible (evens out the acceleration demands of the car), ease off the gas as soon as I see a stop sign or stop light ahead and as I'm coming into an exit on interstates, accelerate gently unless I need to quickly pass someone or get out of the way of another vehicle, and stop the practice of creeping forward in stopped or very slow traffic. Instead, I just stay put, with my engine shut off as hybrids do when the come to a "reg

8/4/2007 12:17:43 PM

We nave a 04 Prius that gets 50 MPG on the highway all the time - with 4 people, AC, full of gear, running 70 mph+, w/our 6'4" son in the back seat! Great car, come on Detroit get with it already! This is not just a fad.

8/3/2007 12:36:02 PM

Don't forget to slow down, too. My vehicle's MPG has gone from about 31MPG to just over 40MPG by doing the following: 1. Switch to fully synthetic oil. 2. Stop using the air conditioner. 3. Drive a maximum of 60MPH. This is a car with an EPA of 23/31. Also, has some excellent advice.

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