Aces Full of Links

Gallons Per Mile

The way we think about things has a strong effect on the conclusions people often come to. This is why you often hear opposing political camps struggling to get people to adopt their names for certain issues -- even the name of an issue can change the way it is perceived by citizens (e.g. pro-choice vs. pro-abortion).

Sometimes, we adopt conventions that don't serve us well when we need them most. Such an issue was covered recently in this NPR story about cars and gas mileage about the difference between "miles per gallon" (MPG) and "gallons per Mile" (GPM). Both are ratios that tell you something about the fuel efficiency of a vehicle. They are reciprocals of each other, so they represent a relationship between the same two numbers.

We're more familiar with MPG because it's the measure that car manufacturers use in their ads. But it turns out that using MPG can mislead people who are making decisions about what car to buy.

Especially when multiple cars are involved, people overestimate how efficient it is to own a really-high MPG vehicle. This website has a quiz that illustrates the problem. When it comes time to replace one of your vehicles, is it better to own two medium-efficiency vehicles, or one SUV and one super-efficient hybrid?

Research showed that most people choose incorrectly, because MPG exaggerates how efficient hybrids are and de-emphasizes the inefficiency of SUVs. When people are shown GPM values for the cars, they more easily understand that they don't need to own a hybrid to really improve their fuel efficiency, they just need to dump their SUV.

I can't say for certain why we cling to MPG as a measure. I don't have the evidence to propound any conspiracy theories, but I will say that it makes sense for a society to adopt and encourage language and metrics that serve them best, instead of allowing industries, corporations or other parties to force language and metrics on us that serve their own interests. Ironically, the government already uses GPM when it sets fuel efficiency standards.

Unfortunately, there's always resistance to new metrics; even ones that serve us better. Some resistance comes from corporations, and some is just sloppy thinking. In looking up this story I can't tell you the number of stupid comments I saw along the lines of "if you're measuring your car's efficiency in gallons per mile, you must have a really inefficient car!" The implication being that you must be using one or more gallons in every mile. This person clearly has not grasped the idea of fractional numbers.

A 32 MPG car uses 0.03125 Gallons Per Mile. A 20 MPG car uses .05 Gallons Per Mile. It's not rocket science.

Then there are complaints like this one from "The Green Geek" at TheTechStop.net :

this seems like one of those "no chance in hell" initiatives. I mean… seriously, what's easier to say… "33 miles per gallon" or "0.0303030303030303030303030303030303… gallons per mile"?

I'm hoping that's a joke; of course our ability to measure gallons per mile does not justify precision out to that many decimal places... 33 MPG is likely rounded off to the one's place. 0.0303 GPM is pretty close to 33 MPG. Using GPM we'd just get used to other round numbers.

The decimal problem is nicely handled by using GPM to actually represent gallons per one thousand miles. 33 MPG becomes 30.3 GPM. That's pretty easy to understand, isn't it?

The benefit becomes even more clear when you ask a question like "How much would I save?" when comparing two cars. If you trade your 15 MPG truck for a 25 MPG car, and you drive 100 miles a week, how much gas will you save each week? (If you answer this question you'll likely convert to GPM first)

It's a lot easier for you to do the math if I ask you the same question in GPM: If you trade your 66 GPM truck for a 40 GPM car, how much gas will you save each week at 100 miles per week? You'll use 6.6 gallons in a week with the truck. In the car you'll use 4 gallons. You'll save around 2 2/3 gallons.

And at $4 per gallon, that's over $580 per year.

If we thought in gallons per mile, we'd be at an advantage as consumers. Wouldn't you rather have that advantage?

---

Extra Notes

For folks who have an approximately hour-long commute, your weekly distance is probably more like 600 miles and your above savings would be closer to $3480. Yikes! Don't drive a truck to work!

I commute about 280 miles per week. Here's a question that we actually faced. I owned an old Saturn. I wanted a Jeep for towing and for geocaching. But I didn't want to commute in it if I didn't have to. The Saturn got about 40 GPM . The Jeep takes 58 GPM or so. Does it cost more to keep the Saturn on the road (and pay insurance and registration), or to get rid of it, at $4/gallon.

It comes to $1048 savings per year. So, if your registration, insurance and your excise tax don't exceed $1048, it starts to make sense to have a truck you only use for towing and the like.

Most people overestimate the efficiency of their vehicles and are encouraged to do so by efficient sounding "highway mileage" estimates from auto companies or mileage counters in their cars which may not be completely accurate. Don't guess, record your odometer reading and the number of gallons when you fill up and do the calculation yourself. Or, you could try out http://mymilemarker.com/ I haven't yet, but they say you can SMS your fill-up information, then come back and check your mileage.

Technorati : fuel, gas mileage, green, mathematics, npr

Posted by James at June 26, 2008 2:56 PM
Social Bookmark del.icio.us Digg it reddit