It seems that many Chevrolet Volt buyers are trading in a Toyota Prius. At the test track, a few of us were wondering: How much gasoline is actually saved on transaction like that? And what sort of swap would have a bigger impact? Let’s find out.

The answer to the first question is relatively simple: Likely not much. In our tests, the Prius gets 44 mpg overall, and after a lot of number crunching we have calculated that Volt owners will be getting the equivalent of 58 mpg (counting electric energy, and assuming 70 percent electric driving). At that rate, in the course of 12,000 miles, roughly a year’s use, a typical Volt will use about 66 fewer gallons than a Prius.

If every Prius buyer (141,000 of them in 2010) bought a Volt instead, they would save the nation 21 million gallons of gasoline in a year. Sounds like a lot until you consider that Americans burn through 378-million gallons per day.

For the second question, we turn again to General Motors. It happens that GM makes the very nice and huge-selling Silverado pickup, which together with its GMC Sierra clone sold about 500,000 copies last year. The last one we tested was a crew-cab LT V8, which racked up a miserable-sounding 14 mpg.

But wait. Chevy also makes a Silverado Hybrid that almost no one is buying. Based on our experience with the similar Chevrolet Tahoe SUV hybrid, we think the Silverado Hybrid should get about 19 mpg overall. What if GM pickup buyers switched en masse to the hybrid?

In a year, each hybrid Silverado would save almost 226 gallons over a traditional truck. Five-hundred thousand of those would amount to more than 113 million gallons of gas per year—five times more than would be saved if every Prius buyer chose a Volt instead. Even if the sales figures were the same, each hybrid Silverado would save 76 gallons more than a Prius-to-Volt switch.

The same principle of diminishing returns holds true even when we use more realistic examples. Switching, say, from a 30-mpg Honda Civic to a 44-mpg Prius saves 127 gallons per year. That’s great, but still 99 fewer than the 226 realized from goosing a gas guzzler from 14 to 19 mpg.

For more on saving fuel, see our guide to fuel economy and our report "Comparing mileage: Not all mpg's are created equal"

-Gordon Hard and Jake Fisher

Updated: 8/9/11 5:15 p.m.