The MPG Myth

As gas and oil prices skyrocket, despite the fact that I see very little change in the San Francisco Bay Area in terms of the types of cars driven or the way people drive - more on that later - people are moving away from gas-guzzlers to more fuel efficient cars, out of necessity.

Still, the math ... or rather the myth ... of MPG may be making them making incorrect choices in terms of cars.

A research paper in the journal Science (subscription required) outlines just how misled we may be. When one uses a modicum of "common sense," it's pretty easy to see the results, even without a subscription to access the full article. I was intrigued when I heard a short blurb on this on the radio and did the math myself, as we are considering replacing my wife's Toyota Echo with a Toyota Prius.

The problem is that most people think an increase of 10 MPG trumps an increase of 5 MPG, no matter what. Let's say you have two cars, and were considering replacing an SUV that gets 15 MPG with one that gets 20 MPG. Or you could replace your mid-size that gets 28 with a subcompact that gets 38. And let's assume you drive the same number of miles in each car. The 10 MPG difference should be better, right? Wrong.

In fact, that 15 MPG uses 6.67 gallons every 100 miles. Meanwhile that 20 MPG SUV uses 5 gallons every 100 miles. Assuming $4 / gallon gasoline you will save $6.67 per 100 miles.

At the same time, the 28 MPG car vs. the 38 MPG car results in a savings of $3.76 over 100 miles. Surprising at first glance, but really, it just requires some math (a good spreadsheet helps!).

Obviously if the miles driven between the two cars is skewed the equation becomes more complex. Where's the breakeven point? In this case it would be calculated by dividing the difference the SUV's savings by the car savings: the SUV "change" saves 1.773 times as much fuel as the car "change."

Breakeven, then, would be when the car drives 1.773 x as many miles as the SUV. So, if the SUV drives 10,000 miles a year, and the car 17,730, that's about (accounting for rounding) when the savings would be equal.

I've uploaded a very simple spreadsheet here that will help you determine your best options, if you are considering replacing one of your cars.

Now, as far as my earlier comment about "how people drive," I still see people typically drive 80 - 85 MPH on Bay Area freeways - and that's with a 65 MPH speed limit. I hope most people have enough common sense to realize slowing down saves gasoline.

In fact, while the actual calculation would differ for each car based on a variety of variables, for most cars the "sweet spot" is in the range of 40-60 MPH. So why do we continue to have a 65 MPH maximum speed limit?

It's true, in fact, that truckers and even airlines are slowing down, on their own. But the public: they just don't get it. And let's face it, if your neighbor is using more gas, it's taking away from your and your descendents.

Oh, and BTW, for all those states with budget problems: a little strict enforcement (none of those "warnings" I see given on the TruTV show Speeders) might really help your bottom line. And don't get me started about how many non-carpoolers are in the carpool lane.

End of rant.