Thursday, August 07, 2008

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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Labels:

At 8/07/2008 5:36 PM,  MeadowLark said...

Interesting.
And there are a lot of variables not listed - like who exactly is paying for my new vehicle!!! :)

I think, perhaps, that might be why some of us still drive an SUV.

At 8/07/2008 9:06 PM,  Katie said...

My husband ran across something similar...and makes me feel better to own a crossover!

At 8/08/2008 9:32 AM,  tina f. said...

Can you replace the 10 mpg with the 50 mpg vehicle? Or is that just not an option?

At 8/08/2008 9:34 AM,  Bobbi said...

Math - it makes my brain fuzzy!

At 8/09/2008 4:42 PM,  Owlfarmer said...

Are there ways of increasing the mileage in the present vehicles? I mean, I've increased my mpg by 4 just by cutting my top speed from 65 to 60 and using cruise control as much as possible. Tune-ups, properly inflated tires, and gentler use seem to be first-order strategies. Changing vehicles should come in as a second-order solution.

At 8/09/2008 4:47 PM,  e4 said...

Yeah, there are quite a few ways to improve mileage in existing cars. Driving slower usually works. If you have a tachometer (RPMs) keeping it below, say, 2500 (or whatever seems appropriate for your vehicle) will make a big difference. Coasting to slow down instead of braking where possible. Some say synthetic oil improves mileage.

Check out various trials by people with regular cars over at Groovy Green here.