## Gallons Per Mile: Its Time May Have Come

July 6, 2011 § 1 Comment

Three years ago Richard Larrick and co-workers at Duke University proposed the logic of using gallons per mile instead of miles per gallon (mpg).  In their paper they pointed out that mpg was not linear and hence not intuitive in making decisions.  Such a decision may be for example when considering replacing one of your two vehicles with more fuel efficient ones.  Vehicle X is a land cruiser with 10 mpg and Vehicle Y is a compact with 25 mpg.  One would assume that replacing Y with a 40 mpg vehicle would save more than replacing X with a 20 mpg vehicle.  Not so.  Not even close.

Assume that you drive 200 miles a week.  Vehicle X would consume 20 gallons of fuel and the replacement would consume 10 gallons.  At \$4 a gallon, the saving would be \$40.

Vehicle Y would consume 8 gallons and the replacement would consume 5 gallons.  The saving would be \$12.

This underlines the character of mpg: it is not intuitive for decision making.  Gallons-per-100-miles is completely linear.  Using that metric it is abundantly clear that the Vehicle A replacement results in more savings.

Three years ago that paper did not catch people’s fancy.  One possible reason is that we have difficulty with a smaller number being more desirable.  Also, gallons per mile would be a fraction; not wanting to deal in fractions forced the use of gallons per 100 miles or some such. The biggest reason was likely inertia and not wanting to replace the familiar.

But now it’s time may have come.  Electric vehicles (EV’s) could force the issue.  The easiest way to compute the “fuel” cost in EV’s is to determine the electricity used per mile and multiply by the cost of electricity.  EV’s are likely to be very similar to each other in the charge used per mile.  This is a huge departure from conventional vehicles, where the fuel efficiency can be dramatically different.  EV differences will primarily be premised upon body weight.  Other than that the variability will largely be in the type of driving: stop and go city driving will be more economical than distance driving, largely due to the regenerative braking systems.  This is of course the opposite of conventional cars and the EPA will have to consider this in their testing.

The capacity of an EV will be measured in kilowatt hours (kWh).  The Nissan Leaf has a capacity of 24 kWh and uses 0.25 kWh per mile, so it has a range of 100 miles for average driving on a full charge.  There you have it.  The kWh per mile is your gallons per mile analog.  For EV’s nothing else makes more sense as a metric because the consumer knows what she pays for electricity at home, and easily computes fuel cost.  In fact the ease of computation is going to be a key to driving the right battery charging behavior.  We expect utilities to incent night time charging by heavily discounting rates at night.  Absent that, significant day time charging could create serious problems for the grid.

Now, one could argue for EV’s to most usefully employ kWh per mile but for conventional cars to remain as before.  Sure, but when being responsive to federal fuel efficiency mandates, the mixed fleet will need to be assessed.  In so doing, the gallons per 100 miles, taken together with kWh per 100 miles for EV’s will allow direct comparison.  This is because we know that a gallon of gasoline has 34 kWh of energy.  So gallons per 100 miles could convert to kWh per 100.  Somehow I doubt the populace will go for this wholesale switch to kWh.  But changing from mpg to gallons per 100 miles will allow an easy computation of fuel efficiency of a mixed fleet and allow direct comparison with EV’s for consumer choice.