Some statistics, like the world population, give you instant perspective on complicated issues. It doesn’t take a lot of math to figure out what percentage of people live in China if there are 1.1 billion people there and 6 billion people in the world.

Miles per gallon sounds like it should be similarly intuitive. But it’s misleading. Consider the following, which was a Car Talk puzzler a little while ago:

You drive a car that gets 50 miles per gallon. Your spouse drives a big SUV that gets 10 MPG. You both drive the same amount. You have the chance to trade in your car for a new one which gets 100 MPG. As an alternative, your spouse can fix the tire pressure in the SUV so it will get 12 MPG. Which will save more gas, going from 50 to 100 MPG, or from 10 to 12 MPG?

The thing which makes this difficult is that MPG measures the inverse of what people want to know. You don’t decide how many miles to drive based on how many gallons are in your tank. You want to go somewhere in particular, and you need to know how many gallons you’ll need. So the useful statistic is gallons per mile, not miles per gallon.

Of course, every car out there gets more than a gallon per mile, so a more reasonable statistic is gallons per 100 miles. The puzzler can then be restated:

Which will save more gas, going from 2 gallons per 100 miles (GP100M) to 1 GP100M, or going from 10 to 8.3 GP100M?

MPG is just a “bigger is better” number. But presented as GP100M, it is clear just how little gas the more efficient one uses in comparison to the gas-guzzler. If the stickers at the car dealer showed GP100M, I’d be able to calculate in my head roughly how much gas I’d need for my weekly commute.

I work about 10 miles from home, so that’s 20 miles five days a week, or 100 miles. If gas is $2/gallon, the 10 GP100M SUV costs me $20/week, versus $4 for the efficient car. If prices go to $3/gallon, the cars cost $30 or $6 per week. (Admittedly, the math happens to be particularly easy in my case.)

My actual car (a 1996 Toyota Corolla) gets about 30-35 MPG. That’s 2.8-3.3 GP100M. So if gas gets up to $3/gallon, I’ll be paying about $9/week on gas.

Here are a few more examples of GP100M:

2006 Toyota Prius: 51/60 MPG; 2.0/1.7 GP100M
2006 Hummer H3: 16/20 MPG; 6.3/5 GP100M
2006 VW Beetle Hatchback 22/31 MPG; 4.5/3.2 GP100M