[Fat Charlie]

It doesn't affect the mpg readings, as such, but it helps if you want your display to be relevant to the tank. My display shows instant and short trip mpg, mpg for the current tank, fuel level and DTE. None of this matters when you write your odometer reading on the pump receipt, but long trip (tank) mpg on the display tells me how my week is going overall and lets me know what the pump receipt is going to say.

All these numbers work in relation to the others (and the car's gas gauge and trip mpg display, if you have one) and lets you get a feel for how accurate all your numbers are: if you can't trust your gauges, what good are they? All of my numbers tell me different things that I want to know and the also help me confirm the other numbers.

So after you get back in your car you spend 30-40 seconds pressing buttons to zero out trip numbers and tell the gauge you filled up. Who cares? That's less time than you "waste" driving less than 80 mph everywhere.