Using MPG (Miles Per Gallon) to measure the fuel economy of a vehicle can lead to some wrong impressions. It is not that there is anything wrong with MPG itself, just that it measures something that we don't normally use. Much better is to think about fuel economy in GPHM (Gallons Per Hundred Miles). Here is an example that shows how easy it is to be confused with MPG.

Assume you have a car that gets 10 MPG in the city and 20 MPG on the highway. Now this car goes on a trip that is 10 miles in the city and 10 miles in the highway. Over this 20 mile trip, what is the average MPG that you got? If you said 15 MPG you are mistaken. It looks like it should be 15 because 15 is the average of 10 and 20. But unfortunately MPG does not work that way. Let us see why not.

Over the first 10 miles in the city your car used 1 gallon of gas because it went 10 miles and gets 10 miles per gallon. Over the next 10 miles on the highway, you car used 0.5 or one half gallon of gas because it went 10 miles and gets 20 miles per gallon. This means your car used 1 gallon plus one half gallon or 1.5 gallons for the whole trip. The total mileage of the trip was 20 miles. This means the MPG for the trip was 20 miles per 1.5 gallons or 13.3 MPG! Not 15 MPG.

What happens if we use Gallons Per Hundred Miles or GPHM instead? To convert MPG to GPHM divide 100 by the MPG rating. The car gets 10.0 GPHM in the city and 5.00 GPHM on the highway. If we average 10 GPHM and 5 GPHM we get 7.50 GPHM. Is that the right answer? If we convert the 13.3 MPG we found before, it comes out to be 7.50 GPHM on the nose.

So you can average out GPHM ratings and you can't average out MPG ratings. This is a perfect example of why we would be better off using Gallons Per Hundred Miles (GPHM) and not MPG when we talk about fuel economy. GPHM works out like we expect, but MPG can be tricky and you can easily fool yourself.

Why does this happen? Well, it is because the two measures are thinking about taking different things as givens or fixed. With miles per gallon (MPG) you are thinking about using a fixed amount of gas and seeing how far you can go. The idea is to talk about the range you can get with a given amount of gallons.

With gallons per hundred miles (GPHM) you are talking about going a fixed distance and seeing how much gas it will use up doing it. Here you are talking about the fuel usage that it will take you to cover a fixed distance.

Now we can see the root of the problem when we try to use MPG for fuel economy. MPG assumes that you are going to use a fixed amount of gas and tells you about the different ranges different cars could get. But assuming you will use a fixed amount of gas is wrong. We want to go a certain distance. We don't say "I will drive until my car uses up 10 gallons." We say "I will drive to work (which is a distance of 10 miles away)." We should use GPHM to talk about fuel economy because the whole point of saving on gas is not to use a fixed amount of gas, but to change the amount and make it lower.

If you want to read more about this, I wrote about it before and the experts explain it really well here. As always, Wikipedia has useful information. And here is a handy calculator you can use to make conversions.

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