I couldn't help but notice numerous comments and diaries over the last few days disparaging the Cash For Clunkers program. Many of the complaints have a common thread that I really feel compelled to dispel; that trading up from a 17 mpg clunker to a light SUV or large car has minimal environment benefits compared to, say, upgrading from your conventional car to a highly efficient hybrid or similar car. Of course any fuel efficiency upgrade is a good thing for the environment and for America's energy security. But I want to take a few minutes to straighten out some commonly misunderstood math. Everyone on Daily Kos is certainly smart enough to understand this math, but the real way of calculating gas savings simply doesn't occur to many people. So join me below for a little math fun! This may help some of you insomniacs get to sleep tonight. LOL.
When thinking of gasoline usage, miles per gallon (mpg) is actually not an accurate way to gauge the benefits of an upgrade. Rather its inverse, gallons per mile, is the direct measurement of what you want to consider. That is how much gas you burn per mile driven. To convert from miles/gal to gal/mile simply divide 1 by the mpg value. With that in mind, consider two alternate car trade-in scenarios:
First Scenario - Someone trades in their 25 mpg sedan for a 40 mpg hybrid. This is a very respectable fuel efficiency increase of 15 mpg. The direct measurement of actual fuel savings is calculated by subtracting the inverse of each number; 0.04 gal/mile (inverse of 25 mpg) minus 0.025 gal/mile (inverse of 40 mpg) yields 0.015 gallons saved for every mile driven. Very nice indeed.
Second Scenario - Now suppose someone trades in their 17 mpg clunker for a 23 mpg light SUV. This is a much smaller fuel efficiency increase of only 6 mpg; less than HALF of the mpg savings from example 1!!! But before you sniff in disapproval, here's the catch. The direct measurement of actual fuel savings is really gallons per mile. 0.059 gal/mile (inverse of 17 mpg) minus 0.043 gal/mile (inverse of 23 mpg) yields 0.016 gallons saved for every mile driven... nearly the exact gasoline savings as the hybrid upgrade in example 1.
The Bottom Line - Now sure, the best upgrade of all would be to go from the 17 mpg clunker right up to the hybrid. That's obvious. But that's not the point here. The point is that Scenarios 1 and 2 yield virtually identical environmental benefits. In other words, the Cash for Clunkers program for people who like larger vehicles can yield serious reductions in gasoline use on par with upgrading from a conventional car to a hybrid... and I sure don't hear many complaints around here about hybrid car tax incentives. That, folks, is why we should celebrate the success of the Cash for Clunkers program; because results matter.