What you might be thinking of, or what someone might have mistakenly called a "fuel reset" button, is the trip odometer reset button. This button resets the counter showing how many miles you've driven since the last reset. It has *no* effect on fuel economy calculations themselves. To see your average fuel economy, you'd typically use the car's built-in computer system (if equipped) or calculate it manually by dividing miles driven by gallons used.
