>You drive 80 miles an hour, you are wasting gas and getting poor mileage.
>
>You are stopped, you are wasting gas and getting very poor mileage.
>
>How do you know what is the speed to drive to get the best mileage?

I think it really depends on your particular car, the terrain, and the temperature.

The only way I get a clue is to track miles and gas purchases. I check my mileage on every fill-up and then reset the trip odometer. Over time I get a pretty good idea of why I get the mileage I do (between 26MPG and 35 in my Kia). I get the best mileage on the highway doing around 60 with the A/C on on a hot day. Turn off the A/C and the mileage goes down by 2MPG (??why??). Cold days I never get over 31. Over 75MPH and I won't get over 30.

YMWV.