I keep up with the milage for my Lincoln LS. I just write down the mileage on the odometer and the amount of gas I pump in to fill the tank. The mileage I calculate for one fillup is not very accurate, given that how full the tank is varies, and the gauge on the pump is not highly accurate, but over a few months it is accurate. My average mileage has fallen about 0.5 mpg since the government has required that the fuel contain 10% ethanol. Ethanol contains less energy per gallon than does gasoline, so this result is what should be expected. This leads me to wonder if the Congress considers this when establishing the fleet mileage requirement. Obtaining an average mileage of 30 mpg for pure gasoline than it would be for 10% ethanol gasoline, to say nothing of E85.