Most cars get their absolute best mileage in the range of a steady 40-45 mph. This is the "crossover point" where the increase in drag overcomes the inherent losses of the engine running at very low power levels. It's also the point where most cars will shift to their highest gear and you want the engine turning as few RPMs as possible for a given speed to get the best mileage. But the peak is very flat, so anything between 30-mph and 55 mph will be very close..... it is only when you start going over 55-mph or so that drag starts to rise enough to seriously affect mileage.
As previous poster mentions, the big key is to avoid using brakes and accelerate lightly so the car stays in a high gear without having to downshift. If terrain is rolling, then letting your speed bleed off up hill and speed up downhill is better than trying to maintain a steady speed. Again, avoid braking at all costs.
- Mark