Last week I mentioned that I've always thought of my car's fuel usage in terms of miles per gallon (mpg). I don't think that's particularly unusual for any car owner to think that way. In fact, until recently, a car's miles per gallon rating for highway and city driving were the prominent numbers displayed on a new car's window sticker. But as more auto manufacturers begin to adopt the new fuel economy labels from the EPA, you're going to begin seeing some new metrics. One of those new numbers is fuel or energy consumption per 100 miles.
The old (and probably more familiar) miles per gallon number is easy to calculate, really. You can do this even while you're standing right there at the pump. Just note the number of miles that you've traveled since your last fill up (resetting your odometer's trip gauge at every fuel stop is the best way to do this) and then divide that number by the number of gallons you just added to the tank to bring it back up to full again. Voila! You've just calculated your car's mpg. Simple, right?
Well as it turns out, even though mpg has been the accepted standard for a long, long time, it's also a bit misleading. How so? Just look at this chart from the EPA -- The MPG Illusion. (The chart is under Section 5: "Fuel Consumption Rate.") It shows the "non-linear relationship between gallons used over a given distance and miles per gallon."
Take another look at the chart, and then wait a minute to let that sink in.
In other words, it's more accurate to compare vehicles by using fuel consumption (gallons per 100 miles) rather than fuel economy (miles per gallon). So, gallons per 100 miles (g/100m) is something you're probably going to have to get accustomed to seeing (and calculating on your own). Here's how you do it:
First, you'll have to know your car's average mpg rating. (I knew that skill would be rewarded somehow.) Let's just say that it's 22 miles per gallon, for our example. But instead of miles per gallon, we need to know how many gallons per mile (gpm) your car gets. So, we take the reciprocal of 22 by dividing 1 by 22 -- and we get 0.045 (that's the gallons per mile rating of our example car). Then multiply 0.045 by 100 (or just move the decimal point two places to the right), and you'll get 4.5 gallons per 100 miles (4.5 g/100m).
22 mpg 1 ÷ 22 mpg = 0.045 gpm 0.045 gpm x 100 miles = 4.5 g/100m
It's not that complicated, really -- and it provides a level playing field when you're comparing new or used cars on the dealer's lot.
And in the case of plug-in hybrid and all-electric cars, you're going to see mileage listed on the new fuel economy labels in kilowatt hours per 100 miles (kW-hr/100m). But how does electricity usage compare to gasoline usage in a more conventional car? You'll have to take a look at my earlier post about MPGe for the answer.