Fuel savings depend on the starting point

The recent revelation that automakers are still not using standard measures for towing capacity brought up the issue of gas mileage, with suggestions from some that the United States switch to gallons per mile (or the European liters per 100 kilometers). L/100km  is clumsy to write or say, but gives a more accurate impression of which vehicles will save more fuel.

Americans have, looking at miles per gallon, tended to emphasize high speedway mileage (40 mpg highway is the hot sales point), when raising city mileage may be more rewarding.

Gaining just one mile per gallon (1 mpg) on a heavy duty truck that averages 15 mpg will buy 4 gallons per thousand miles — around $180 per year. That same 1 mpg gain on a Dodge Dart-class car is responsible for just 0.6 gallons per thousand miles — around $27 per year. Likewise, adding 1 mpg to the city cycle will generally save customers far more than adding 1 mpg to the highway cycle; if you have a car like the Chrysler 300C V6 (19/31 mpg), a 1 mpg increase to the city cycle will save around 2.5 gallons of fuel per thousand miles, but 1 mpg more on the highway cycle will only save about 1 gallon per thousand miles.

The average car is driven around 12,000 miles per year; gasoline prices have been ranging from around $3.50 to $4.00 per gallon over the last five years (the US expects gasoline to be around $3.55 per gallon this year).  Gaining 3 mpg on a 15 mpg truck, over a year, saves $500 a year. On a highly efficient small car, it would save just $77.

These calculations go a good way towards explaining why the first Chrysler vehicles to get a serious, systemic gas-mileage workover in recent years were the Ram series; they also explain the attraction, to numbers people, of  “fuel per distance” measures. While it’s intuitive to have numbers where “higher is better,” miles per gallon is misleading in that saving 2 mpg results in far different fuel use depending on where one started.