Monday, June 30, 2008

How much fuel is that?

Judging fuel efficiency using MPG apparently leads most of us to misjudge the benefits of increasing fuel efficiency for the least efficient cars.

Miles per gallon tells us how many miles we can drive on one gallon of gas. But who has a sense of their gas usage?

What we do have a clear sense for--what we most clearly experience--is how many miles we drive. So the measurement that best matches how we intuitively measure our fuel use is how many gallons it takes to drive a certain number of miles (gallons per mile rather than miles per gallon).

The economists who conducted these experiments found that the MPG measurement is misleading because people don't realize that trading in a car that gets 18 mpg for one that gets 28 mpg is a bigger jump in fuel efficiency (twice as big!) than trading in one that gets 34 mpg for one that gets 50 mpg. The implication for policy is that increasing the fuel efficiency of behemoth gas guzzlers is more effective than incrementally tweaking hybrids.

This would be a great problem for a critical thinking course!

1 comment:

Noumena said...

Because I've been reading ch. 9 of Kitcher's Advancement of science (it's all mathematical-neoclassical economic models of scientific communities), I've gotten back in the habit of doing spontaneous calculations.

Like the other day, when, while driving around doing errands, I was using estimated miles per gallon and estimated average gas prices to calculate cost in dollars per mile for gas for various kinds of cars. At California rates ($4.50/gallon), the SUV I'm borrowing (20 miles/gallon) costs 22.5 cents per mile to drive around. If the fuel efficient were boosted to 30 miles/gallon, that cost would drop to 15 cents per mile, which is a drop of 33%. Pretty nice!

A lot of folks here in rural Northern California (where they drive pickups and SUVs because they actually need the cargo space and often need the four wheel drive) are savvy enough to compare the additional cost for a hybrid engine, which is somewhere around $5,000. Assuming you own the car for about 120,000 miles (which is roughly how long my family considers a Honda to be reliable for), that's an additional 4.2 cents per mile. To be a better investment, Honda's hybrid SUV would have to be at least 25 miles per gallon (average over the 120 thousand mile effective lifespan), and gas would have to stabilise at $4.50/gallon. At $5/gallon, the hybrid engine would have to be nearly 28 miles per gallon. Hence my pessimistic mother buying a conventional SUV rather than a hybrid.