Fuel efficiency puzzle

Okay, here's a little pop quiz for you.

Let's say you have two vehicles. One is a big old SUV that gets 10 miles per gallon. The other is a family sedan that gets 25 miles per gallon. You drive both about the same number of miles.

Rising fuel prices have made you decide to upgrade one of the two vehicles to save on gas. You scour the auto listings and you find two possibilities. One is a newer crossover vehicle that could replace your SUV. It gets 20 mpg. The other is a high-efficiency hybrid sedan to replace your current car. It gets 50 mpg.

[For the purposes of this discussion, we're leaving out complicating factors such as embodied energy, vehicle capacity, maintenance costs, alternative solutions, and creative answers. The point of this is to illustrate that our intuition can be way off for something that is becoming increasingly important. I am of course tipping my hand that this is a trick question, but I bet you'll think the right answer is either wrong or confusing. And feel free to imagine km per litre, rods per hogshead, or whatever units you prefer. The units don't really matter.]


So which is the better strategy:

A. Replace the 25 mpg vehicle with one that gets 50 mpg
B. Replace the 10 mpg vehicle with one that gets 20 mpg

Take a minutes to think this over.

[ scroll down for explanation ]
















Are you convinced your answer is right?



Did you pick A?
After all, you'd save 25 miles per gallon instead of ten!


The correct answer is B. By a longshot, actually.

Take a look at this chart:

Gallons used per 1,000 miles driven
	Old	New	gallons saved
Car	40	20	20
SUV	100	50	50

So in this scenario, you actually cut two-and-a-half times more gas by switching the SUV rather than the car - 50 gallons saved vs. 20.

For the whole picture, we can total the two combinations of vehicles (new car + old SUV vs. new SUV + old car) to show the first combination uses one third more - 120 gallons vs 90 gallons.

The trick is to invert the common miles-per-gallon measure into gallons-per-mile. Or maybe gallons-per-1000-miles, for easier readability. The fuel-per-distance gives a more accurate and intuitively clear measurement of fuel efficiency.

Obviously I'm leaving out a thousand variables. But I thought it was interesting.

Science Daily has a great article on this.

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