The best (blog and academic) work that I’ve seen on why we should work with gallons per mile (gpm), rather than miles per gallon (mpg), comes from Duke Professor Rick Larrick. He blogs at MPG Illusion and GPM Calculator. As we consider the Clunker of a Deal (otherwise known as Cash for Clunkers), it is worth considering why it would have been far better to use how many gallons per mile (100 miles) saved as the basis for determining the “cash for clunker” payment.
Why is gallons per mile? Moving from 15 to 20 mpg is, statistically, the same as moving from 30 to 40 mpg (each a 33% increase in fuel efficiency) in one way, the way most people think, but this doesn’t work out to the same savings in gallons per mile. To make life easier, let’s us consider 100 miles for comparison rather than 1 mile.
15 mpg = 6.666 gallons per 100 miles (gp100m); 20 mpg = 5 gp100m or a savings of 1.66 gallons
30 mpg = 3.333 gp100m; 40 = 2.5 gp100m or a savings of .83 gallons per 100 miles driven
In other words, that statistically same (one direction) improvement actually differs by a factor of two considered another way.
Consider an “average” driver of 12,000 miles per year. Moving from a 15 to a 20 mile per gallon vehicle would save 200 gallons per year (falling from 800 to 600 gallons per year) and reduce carbon dioxide emissions by about 5000 lbs per year (counting the systems costs for delivering the gasoline to the vehicle in addition to the straight gallon of gas CO2 emissions). Moving from a 30 to 40 mpg vehicle would save 100 gallons.
For that 30 mpg vehicle owner to match those the 15 mpg vehicle owner’s switch to a 20 mpg vehicle, the new vehicle would have to hit 60 mpg. That is, moving from 400 gallons / year of fuel use to 200.
Of course, what we would like to seriously incentivize getting that 15 mpg vehicle owner either out of a car (if appropriate) or into a 30 mpg (400 gallons/year savings) or 40 mpg (500 gallons/year savings) or 50 mpg (560 gallons/year savings) vehicle.
In any event, if we use a a gpm (or gp100m) standard, the actual savings in terms of total fuel use become self evident and provide a far better basis for policy-making.
2 responses so far ↓
1 Is it worth $2000 to the country to increase the MPG of one vehicle by say 10 mpg? // Aug 10, 2009 at 10:14 am
[…] 15 to 25 mpg that initial information is providing re the CARS program and translate it to the more meaningful gallons per 100 miles (gp100m): This cuts from 6.66 to 4 gp100m. Or, for the average 12k miles / year of the average car, this is […]
2 From Cash-to-Clunkers to The Longer Term // Aug 11, 2009 at 1:38 pm
[…] target as the CAFE standards tighten in years to come. (It also means that it relies on the misleading “mpg” rather than more meaningful gallons per mile.) The Senators have a feel for the politics. Let’s seize on CARS Program’s success and […]