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:
Old vehicles
Reported Average MPG = 15.8
Average GPM = 0.064082517
Actual Average MPG = 15.6049
New vehicles
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.
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Friday, November 6, 2009
Final Cash for Clunkers Numbers (But with Doubts)
The final Cash for Clunkers numbers are in:
Old vehicles: 15.8 MPG on average
New vehicles: 24.9 MPG on average
That saves about 2 gallons of gas every 100 miles of driving, or 2 tons of CO2 every 10,000 miles of driving.
There is an AP story ridiculing the fact that some pick up trucks were traded in for other pick up trucks with essentially the same MPG (a 15 MPG truck for a 16 MPG truck), but that loophole was obvious from the start. It was built in by design to support Detroit. The Feinstein/Schumer/Israel contingent tried to hold out for good size increases in MPG, but had to compromise with the Stabenow contingent. Bare minimum truck trade ins was the disappointing concession. The time to publicize this flaw was in May. (As we noted in June, "The final bill has decent MPG improvements for cars (4 MPG and 10 MPG)--enough to "payback" the carbon released in producing the car--but not for large light duty trucks. We believe a tiered system based on GPM and requiring larger gas savings would have been better for reducing CO2 emissions.")
It's a little late now to lament it.
However
The numbers on the NHTSA site inspire no confidence, so all of these conclusions, from my perspective, are in doubt. For example, the NHTSA reports an overall new MPG level of 24.9 based on the following data:
59% new vehicles with an average 27.9 mpg
34% new vehicles with an average 21.6 mpg
7% new vehicles with an average 16.2 mpg
Amazingly, they have averaged the mpgs to get their final figure. Of course, they need to take the harmonic mean, which requires converting the mpg figures to gpm before calculating the final average. The harmonic mean is 24.2 mpg. So which is the real final figure: 24.9? or 24.2?
Update: This post examines the data more closely and finds that the actual MPG of the new vehicles is 23.8.
Old vehicles: 15.8 MPG on average
New vehicles: 24.9 MPG on average
That saves about 2 gallons of gas every 100 miles of driving, or 2 tons of CO2 every 10,000 miles of driving.
There is an AP story ridiculing the fact that some pick up trucks were traded in for other pick up trucks with essentially the same MPG (a 15 MPG truck for a 16 MPG truck), but that loophole was obvious from the start. It was built in by design to support Detroit. The Feinstein/Schumer/Israel contingent tried to hold out for good size increases in MPG, but had to compromise with the Stabenow contingent. Bare minimum truck trade ins was the disappointing concession. The time to publicize this flaw was in May. (As we noted in June, "The final bill has decent MPG improvements for cars (4 MPG and 10 MPG)--enough to "payback" the carbon released in producing the car--but not for large light duty trucks. We believe a tiered system based on GPM and requiring larger gas savings would have been better for reducing CO2 emissions.")
It's a little late now to lament it.
However
The numbers on the NHTSA site inspire no confidence, so all of these conclusions, from my perspective, are in doubt. For example, the NHTSA reports an overall new MPG level of 24.9 based on the following data:
59% new vehicles with an average 27.9 mpg
34% new vehicles with an average 21.6 mpg
7% new vehicles with an average 16.2 mpg
Amazingly, they have averaged the mpgs to get their final figure. Of course, they need to take the harmonic mean, which requires converting the mpg figures to gpm before calculating the final average. The harmonic mean is 24.2 mpg. So which is the real final figure: 24.9? or 24.2?
Update: This post examines the data more closely and finds that the actual MPG of the new vehicles is 23.8.