Cash-for-clunkers gallons-per-mile calculations

By Felix Salmon
May 8, 2009

Ryan Avent and the MPG illusion both examine the “cash-for-clunkers” bill from the perspective of how much in the way of carbon emissions will actually be saved when someone takes advantage of it. But there are a few sums missing in these posts, so I thought it would be worth filling them out. Here’s Ryan, for instance:

For passenger cars, the incentive is reasonably ambitious: those moving from less than 18 mpg to better than 22 mpg qualify for $3,500 for a four mpg improvement and $4,500 for a 10 mpg improvement.

But standards quickly decline as you move up in size. For SUVs and light trucks one qualifies simply by moving from below 18 mpg to above 18 mpg. A $3,500 voucher is available for an improvement of just two mpg, while a mere five mpg improvement gets you the full $4,500 available.

The full table is here, but only in MPG form. In terms of gallons of fuel used per 100 miles, things look a bit different. Here’s how things work out in useful gallons per mile, rather than silly miles per gallon.

To get a $3,500 voucher by trading in a car, you need to move from 18mpg to 22mpg — which is an improvement of 1 gallon per 100 miles.

To get a $3,500 voucher by trading in a small SUV/truck, you need to move from 16mpg to 18mpg — which is an improvement of 0.7 gallons per 100 miles.

To get a $3,500 voucher by trading in a large SUV/truck, you need to move from 14mpg to 15mpg — which is an improvement of 0.5 gallons per 100 miles.

To get a $4,500 voucher by trading in a car, you need to move from 12mpg to 22mpg — which is an improvement of a whopping 3.8 gallons per 100 miles.

To get a $4,500 voucher by trading in a small SUV/truck, you need to move from 13mpg to 18mpg — which is an improvement of 2.1 gallons per 100 miles.

To get a $4,500 voucher by trading in a large SUV/truck, you need to move from 13mpg to 15mpg — which is an improvement of 1 gallon per 100 miles.

So Ryan’s absolutely right: the criteria for SUVs are much weaker than the criteria for trucks. Why do you need to improve by 3.8 gallons per 100 miles in order to get the $4,500 voucher on a car, when you can improve by just 0.5 gallons per 100 miles in order to get a $3,500 voucher on a large truck? It doesn’t make a lot of sense.

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