When you think about it, Miles Per Gallon is really only a useful metric if you consider it in relative terms. You know that a car getting 35 MPG is generally accepted as being more environmentally-friendly than a cart that's getting, say, 17 MPG. Honda Civics tend to consume less gasoline per mile traveled than Hummers do, and since Civics conserve gasoline relative to Hummers, we look at them favorably.

By if you're driving in a car that has a MPG meter on it, so that at any given moment, you can glance down at the dashboard and know just how many MPG you're getting, given the conditions of the road and the way you're driving.

MPG helps in comparisons between cars, but does it really help you to know exactly what MPG you're getting from your car at any given time? Presumably, the gauge is there so people will be able to adjust their driving behavior according to it, to lessen their impact on the environment. But the MPG measurement doesn't communicate that...it's just a number. It helps us make fuel-economy comparisons between cars, and between the highway and city driving of a single car...but beyond that, it doesn't tell you anything useful. (I suppose it is a good red flag that tells you when there might be a problem in the engine.)

If your car could tell you something besides just a number, what would you want to hear?