Originally Posted by Grabber Blue
The computer isn't the most actuate thing in the world. When going between fill ups from empty to empty write down the actual milage of the car when you first fill up till your next fill up. See how many miles you did and you'll get a better idea of your avg mgp/ miles per tank. This way you can see if there is anything wrong with the car.
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Manually calculating isn't any more accurate than the computer. You are assuming that every time you fill up the pump pressure is exactly the same, the delivery of fuel is coming in at the same rate, the temperature is exactly the same, and that you are only putting exactly the same amount of fuel as you burned. Both the manual calculation and the computer are estimates - and I find both are so close that it really doesn't make a difference. Just clear the computer at each fill up and start from "0" and it is just as accurate as your pencil and paper system (maybe more since it is measuring the gas that goes through the injectors)