Letter: Calculate your miles per dollar
Given the instant variability of today's gasoline prices I suggest that drivers start making a new calculation - miles per dollar - instead of Miles Per Gallon. No car or driver's MPG is constant. It changes, based on what gasoline you use, how you drive, what car you have, and many other factors.
Arguing or bragging about your MPG is comparing your apples to the other guy's oranges. But, if you keep track of how many miles you drive between each fill-up (many cars have a resettable odometer) and divide that mileage by the cost of the gas you just bought, you come up with a Miles Per Dollar figure (MP$) that you can compare with yourself at your next fill-up, or with your neighbor with the SUV or the Prius.
It also generates a number that you can directly relate to, for example, you might say, "every five miles I drive costs me a dollar in gas." Trying to increase your MP$ might encourage more conservative driving as well as helping your wallet.