On Fuel Efficiency

An interesting study came out showing that people are confused by the MPG rating on cars. Obviously people can correctly figure out that a higher MPG number is better than a lower one, but the issue came when selecting upgrade paths.

Say a family has two vehicles. One is a truck that gets 12mpg, and the other is a car that gets 25mpg. This family decides it wants a new vehicle and it wants to save as much money as it can on gas (and help the environment). Both the car and truck are driven roughly equally. Do they get a new truck that gets 17mpg (an increase of 5mpg over the current one), or spring for a Prius to replace their car (going from 25mpg to 45mpg, an increase of 20mpg)?

If you chose the Prius, you must hate the environment, because the new truck would be the much better choice.

Why? Because think of it in terms of the actual gas saved. A 12mpg truck driven 10,000 miles a year would use about 833 gallons. A 17mpg truck would use 588 gallons, or about 245 gallons less.

A 25mpg car driven the same distance would use 400 gallons, and that Prius would use 222, or 178 gallons less. Thus, (taking 245-178) buying the new truck would save 67 gallons more per year than buying the Prius.

The study showed that this could be solved by simply giving people a better set of numbers to think about. Instead of miles per gallon, give then gallons per mile. And, since gallons per mile is a tiny number and people seem to abhor tiny numbers (audio file), make it really easy by giving it to them in gallons per 10,000 miles.

If you do it this way, our previous example becomes immediately obvious.

The same family has a truck with a rating of 833 (gallons per 10k miles) and a car with a rating of 400. They are trying to choose between a new truck with a rating of 588 and a Prius with a rating of 222. If they buy the truck their combined ratings would be 988 (400+588). If they choose the Prius, their combined ratings would be 1055 (833+222). Obviously the new truck route would be more efficient (988 is less than 1055).

An interesting example of how numbers can be counter-intuitive at first glance. When stated in MPG, the 20mpg increase given by the new Prius seems far better than the measely 5mpg increase given by the new truck, but the reality is the opposite.

You can go take an online quiz made by the people who did the study if you want to test yourself. The overall lesson: think in gallons per mile instead of miles per gallon. It's even easy to do the conversion yourself. Just take 10,000 and divide by the MPG rating of the car. That will give you gallons per 10,000 miles.

For instance, my car gets about 34mpg. If I take 10,000 divided by 34, I get 294. Buying a new Prius (with a rating of 222), would save me about 72 gallons every 10,000 miles. At current prices, that is about $280. Obviously $280 isn't worth buying a whole new car for, and thinking about it this way makes it very apparent.

This should also make another point very clear. The easiest way to save a lot of gasoline is to focus on the biggest vehicles. If the average semi-truck gets 9mpg, and somehow we improved that to 11mpg (just a 2mpg improvement), it would save more gas than replacing a car with a Prius (moving from 25mpg to 45mpg, or a 20mpg improvement). Add to that the fact that semi-trucks drive many more miles on a yearly basis than any car owner does, and you have to wonder why people aren't clamoring for hybrid semis. Having a fleet of hybrid semis getting 14mpg would do way more for the environment than converting every car in America into a Prius.

Of course, I'm still holding out for plugin hybrids. Bring on the electric cars!