The MPG Illusion: Comparing Fuel Efficiency Can Be Tricky

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While researching the benefit of swapping one of my 21 mpg cars for a 31 mpg Honda Fit, I came across a site called the MPG Illusion. The easiest way to illustrate their point is via a quick quiz.

Pop Quiz
The full quiz and explanation is here. But the very core of the argument can be summed up below. Assuming that both cars are driven 100 miles per week:

Without breaking out the calculator, you might think that having 50 mpg and 10 mpg together might average out to about 30 mpg, beating out the two 20 mpg cars. The problem is that using miles per gallon is not intuitive. Why?

Miles per gallon (MPG) is more useful for things like calculating the range of your vehicle.

Gallons per mile (GPM) would be better for estimating the actual cost of driving your car, since gallons is directly proportional to dollars spent.

Okay, a little math. Getting 10 miles per gallon is the same as saying you use 10 gallons every 100 miles. 20 mpg means 5 gallons every 100 miles. 50 mpg means 2 gallons every 100 miles. So in one week, the Option 1 uses 10 gallons per week. Option 2 uses 12 gallons per week. Option 1 wins!

If you remember gallons = $$, this chart below shows (also from MPG Illusion), the savings you get from going from 10 mpg to 20 mpg is a lot greater than going from 30 mpg to 40 mpg. In fact, going from 16 to 20 mpg can save as much gas as the shift from 31 to 50 mpg.

Higher mpg numbers are still better, but the benefit is diminishing. Going for the hybrid might get you the most “green” points, but you might be getting most of the benefit for a lot less money by simply switching to a more affordable car with a decent mpg bump. Is it almost used Honda Fit time? Run the numbers for yourself at this GPM calculator.

(Does this make the “Cash for Clunkers” program more palatable? I don’t know, they could still raise the minimum improvement amounts.)

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  1. The correct way to “average” MPG (just like any other rate) is to use the harmonic mean. So what was wrong with the “average” that you took is that the arithmetic mean is inappropriate when comparing rates.

    If two cars are driven the same and have MPG of x and y, then the total MPG you will get is 2/(1/x + 1/y).

    So in option 1 you get 2/(1/20 + 1/20) = 2/(1/10) = 20 (obviously).
    In option 2 you get 2/(1/50 + 1/10) = 2/(3/25) = 50/3 = 16.67.

    So yes, option 1 is better.

    If they are driven different amounts, then you just need to weight by the proportions.

  2. I miss the days you could get great mpg on a great lower priced car. I had a Honda Civic hatchback in the ’80s rated at 52 MPG (and it seemed to do a little better than that). Now you rarely even get that king of mileage with hybrids.

  3. Stewie Griffin says


    You can blame government mandates for emissions for the decline in MPG on cheaper cars.

    I wish somebody would do the science/math on what effect emission standards are really having on vehicles.

    Strip all those requirements off, and we could have a car that would get 85mpg easily. Reduce emissions by getting much better mileage, not be choking performance with expensive mandated technologies that reduce mpg.

    If I can drive a car 3 times further on 1 gallon of fuel, then I effectively reduce emissions by 2/3 for that same gallon. Sure your hybrid or other emissions controlled vehicle may also put out the same emissions, but you’ve driven only 1/3 the distance. So where is the benefit?

  4. Yeah, I got a lot of questions from people when I switched from my moderately fuel efficient car (29-30 mpg) to a medium sized SUV (23-24 mpg). The monthly fuel cost increase is hardly noticeable, and now I’m driving something comfortable, safer, and better in just about every other way. Plus, I got a great deal on the SUV because people were freaked out about $4 gas last summer.

  5. I work in the Energy Industry with Gas Turbine power plants. The measure we use is called Heat Rate, which is essentially the reciprocal of MPG ie GPM. The problem with this measure for cars is that it gives an ugly number….example 35 mpg = .0286 gpm.

  6. Emissions aren’t the only thing dragging down mpg; you have all that extra weight from reinforced doors for side impacts, airbags, etc. (About 125 extra pounds for cars according to and these are ’01 numbers). Though seriously, after visiting some foreign countries you’ll be glad for emissions requirements.

    The rapidly diminishing returns of fuel efficiency really shows we need to get out of the car culture. When gas can be close to $3 in some places in the middle of the worst recession since the Great Depression you really have to wonder what the cost of gas might be once the economy recovers.

  7. Or you could express fuel economy in gallons per 1000 miles, e.g 28.6 gpkm.

  8. In fact, this is what is already done in Europe. There, gas consumption is measured in “liters per 100km” (so a smaller number is better). Growing up in Germany, 8 l/100km was considered normal, above 10 was considered poor, and 3 l/100km is the elusive goal everyone is talking about.

  9. Stewie Griffin, You are correct in that government mandates are responsible for a decline in MPG (at least partially responsible), but its not emissions standards that are responsible … its safety standards.

    Emissions technology improvements include advances in fuel injection, vapor reclamation, catalytic converters, sensors, and improved combustion, which result in little to no added weight or MPG penalty.

    Safety improvements have resulted in the addition of several hundred pounds of equipment that weren’t around in the 1980s: airbags, sensors, stability control, safety cages, and brake control mechanisms.

    But the real reason for reduced MPG is simply that car manufacturers keep increasing the size of their models with every new iteration, which is why my 1993 Honda Accord is roughly the same size as a 2009 Honda Civic or why my old 1981 Honda Civic is smaller than a 2009 Honda Fit. For some reason newer equates to bigger and bloated, resulting in more weight and less MPG.

  10. kilomile, lol

  11. Or to present the information in a slightly more tangible way:

    Annual cost of gas, as MPG increases.

  12. Brian Griffin says


    The benefit is that by making the tradeoff between using more fuel but creating less emissions, you reduce the amount that you are “poisoning” yourself by breathing the toxins created by car engines. I grew up near Los Angeles in the 80’s, and we bought a 1980 civic after the gas crisis. But driving through LA, you could not see across the sky and the air smelled terrible due to the smog.

    The argument about creating the same emissions by driving 3x as far with a more efficient car doesn’t play, because the majority of people don’t determine how many miles they drive based on the cost of their fuel. They still need to go to work, shop, etc.

    The only thing that I think has had a significant impact on overall fuel consumption, has been the price of gas, which is also what has always been the main market driver for fuel efficiency. Gas prices created the market for the Civic’s in the 1980’s and the Prius’s now. Otherwise, people love their SUV’s.

    Your right, we could have an easy solution to the 100 mpg car, but the sky would be brown.

  13. Interesting post! I have to admit, seeing numbers automatically makes my brain say, “No! Stop!”, but it is important to pay attention to these little details when buying a car. In addition to gas mileage, there are so many costs to consider to make sure you get the best buy. But, by harnessing The Power of Small, or paying attention to those small details, you can make sure you get the right one.

  14. Interesting. I also believe in not getting carried away completely by the gas prices but also give a thought to the gas volume. Here’s another post to give you a different view:

  15. I didn’t get a chance to comment on your last post, but this is exactly what I was going to say. When the EPA revamped the MPG ratings a couple years ago (to make them more realistic) there were very few people advocating for switching to GPM, but I was one of them. It is a linear scale the makes comparison much more straightforward.


  16. Miyoshi,

    I like your examples but being a Honda freak, I think you’re off by a year in that a 1994 Accord is roughly the same size as a 2009 Civic. I’ve owned both so I’d say you are right on. I’ve driven a ’93 it was way bigger than the ’94 & I remember the outrage in ’94 when they shrunk it, however I’ll never own an Accord again because it is too big now.

  17. The vehicle size thing is most noticable in pick-ups. You could fit a 1989 F-150 in the bed of a 2009 F-150 and still have room for a couch.

    I agree that the real issue here is a vehicle based society – people have come to live in their vehicles, hence drive-thru everything. And we wonder why we have obesity problems. You go from the couch at home to a chair in the car to a chair at work back into the chair in your car, to the couch, then to bed. The most walking we do is behind our lawnmower on Saturdays while cutting the grass – which we wouldnt have so much of if we built at higher density and clustered our open space.

  18. Miyoshi,

    emissions requirements do indeed reduce MPG directly. Emissions requirements prohibit lean burn operation which is what was likely employed in Strick’s honda.

    However, I think we are better off without the bad chemicals in the air.

  19. Surprised not a single prius owner has piped up in this conversation. Sure, it’s an expensive vehicle, but with the lower fuel bills combined with the lower maintenance costs (transmissions last 350K+ miles, as do batteries) and significantly higher resale value (I sold my ’06 in ’08 for $2K more than I purchased it for) it makes economic sense. If you get HOV lane privileges you can also factor reduced commute time into your cost of ownership equation.

    Anyway, this was enlightening, as I see my mileage efforts aren’t as much of a savings as I was thinking. I can get 70MPG average by driving very carefully (timing to avoid red lights, minimizing air conditioning, combining trips and most of all braking only when needed). Still that 70MPG rate is not as much better than my lead-footed wife’s 48MPG “just drive it” strategy as I was thinking. Still an entertaining exercise though.

    It’s a really nice car that’s definately worth a look if it’s in your budget. Much bigger than it looks inside, and the hatchback with fold down seats is really convenient. I get about 20 8-foot long 2x4s in the back with plenty of room to spare. Only the honda fit compares, IMO.

    RE: Car size bloat: It’s just packaging, as americans get bigger so do our houses and cars. It helps us feel OK about our obesity. (Just like “vanity sizing” in clothing)

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