Given the doubts I expressed in this earlier post, I downloaded the NHTSA data and calculated the harmonic mean for the old and new vehicles in the 2009 Cash for Clunkers program.
The harmonic mean prevents the MPG Illusion by first converting all car MPG values to gallons per mile (GPM), averaging GPM, and then converting it back to MPG.
Here are the results:
Reported Average MPG = 15.8
Average GPM = 0.064082517
Actual Average MPG = 15.6049
Reported Average MPG = 24.9
Average GPM = 0.041966565
Actual Average MPG = 23.8285
Well, that final actual MPG figure is a full mile per gallon below the reported figure of 24.9.
That 1 MPG difference translates to 1.2 million tons of CO2 by the time those 677,000 new vehicles drive 100,000 miles.
The supplement to the 2008 Science article discusses an "averaging illusion" example.