I had been wondering about the gas mileage, as displayed on my dashboard. I was supposed to be getting 30 mpg on the highway, but the gauge was firmly stuck around 21. I thought the displayed number was an average of the last x miles, or somthing. Last night, while I was on the highway, I hit the reset button, and the number changed. It read in the low 30's, for the rest of the trip.

So now I know that it's an average of your miles per gallon since it was last reset. And I can test it, with my next tank. And I can more easily check my mileage under certain conditions.