As speeds increase, so does wind resistance. Rolling resistance is, IIRC, pretty constant for all speeds. However, we can also get better mileage in hgiher gears. So, generally, the best mileage should be when you are in the tallest gear your vehicle has, fifth gear when you have a five speed transmission for example, and are travelling at about the slowest speed at which the engine seems to be comfortable, not lugging it.

The other thing that sves mileage when driving, not using the pedal that purposefully slows the car down. brakes are bad for gas mileage, although use of them may be good for your health and the body of your car.