Skip to main content Skip to navigation

Saving Gas Upside Down

Posted by | August 30, 2008

If you’ve been feeling despair about your gasoline bills this year, you have me for company. Having a root canal is looking like fun compared to filling up a gas tank. It is no wonder lots of us are examining our driving habits and our vehicles with new intensity.

Over the years we’ve all grown used to thinking about fuel efficiency in miles per gallon, which is the way the EPA reports it. My sedan gets 27 miles per gallon, your compact gets 31 miles per gallon, and so forth. That’s valuable information. But if you own two or more cars, standing those numbers upside-down could help you save gas.

Here’s what I mean. If your family owns two vehicles that you drive about the same distance, and if you are considering turning one in on a more fuel-efficient model, then the most useful way of comparing vehicles is actually gallons per mile rather than the familiar miles per gallon. With the help of an example, let me show you how it works.

Let’s say – just hypothetically speaking – that your family owns a Hummer that gets 10 miles per gallon and a compact car that gets 33.3 miles per gallon.  And let’s say you drive each vehicle 300 miles per week.

For the Hummer, you are buying 30 gallons of fuel per week (because 300 divided by 10 is 30 gallons). For your small car, you are buying 9 gallons of fuel per week (because 300 divided by 33.3 is 9 gallons).

Say that you’d like to turn in one of the vehicles for a more fuel-efficient model. Your teenagers want to turn in the Hummer for a smaller SUV that gets 20 miles per gallon and looks great. Your spouse wants to turn in your compact for a hybrid that gets 43.3 miles per gallon. In other words, both choices mean you will get 10 additional miles per gallon from one of your vehicles.

Which should you choose? The increase is 10 miles per gallon in each case, so maybe you’d think there’s no difference. Might as well flip a coin? On the other hand, the 43.3 mpg figure looks great, so maybe your gut says to go with the hybrid.

If you want to save gas, here’s how it works out.

If you turn in the Hummer for the smaller SUV, you’ll use 15 gallons each week to fuel the new vehicle (because 300 divided by 20 is 15 gallons). That’s a savings of 15 gallons over where you were with the Hummer.

If you turn in your compact for the hybrid that gets 43.3 miles per gallon, you’ll use 6.9 gallons of gas each week to fuel the hybrid (because 300 divided by 43.3 is 6.9 gallons). With your permission, I’ll round 6.9 to 7 gallons, to keep the numbers simple. So, that’s a savings of 2 gallons compared to the 9 gallons you were using for your plain-vanilla little car.

Clearly, you’ll save a whale of a lot more gas by going with the SUV to replace the Hummer. The allure of the hybrid’s fantastic EPA rating is sweet, to be sure, but don’t let it confuse you about which is your best option under the circumstances.

I recently bought a vehicle to replace one of two that are in my personal fleet. Oddly, perhaps, I chose to retain the more inefficient beast I own, my 1987 pickup with a V-8 engine. But I did so because I drive the truck less than 50 miles per month, so it’s inefficiency isn’t really a significant issue. I also happen to believe, as a matter of deep spiritual conviction, that a geologist should be separated from her truck only by the angel of death. But that’s just me.

Here’s the bottom line: If you drive two vehicles about the same amount each month, take the time to do the gallons per mile analysis of your situation following the pattern of the example above.  Your reward will be making better decisions when looking at replacements for what you’re driving today.

By the way, if you want to disorient a young car salesman, do your gallons per mile calculations on some scrap-paper right on the showroom floor. If you can do arithmetic the old way (by hand, imagine!), the disorienting effect is even greater.

No matter what, good luck at the gas pumps.