>>You drive 80 miles an hour, you are wasting gas and getting poor mileage.
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>>You are stopped, you are wasting gas and getting very poor mileage.
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>>How do you know what is the speed to drive to get the best mileage?
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>I think it really depends on your particular car, the terrain, and the temperature.
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>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.
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>YMWV.

I tend to check the tach. If it's sitting around 1500 or 1600 when I'm in the proper gear for the speed, then I think I'm using gas as efficiently as I can. I find that if I stick to the speed limit, it works out pretty well. Plus, because I drive a stick shift, it's easy to do a lot of coasting on hills. Of course, that's city driving. On the highway, the tach runs a lot higher and gas gets devoured a lot faster.