Depends on the OD ratio/ final drive ratio and the power band of the motor. You get the best gas mileage in the highest gear at the
lowest speed (rpm) that creates the most vacuum. Correct me if I'm wrong here, but I'm not. Vacuum level is inverse to the load demand on the engine, this takes into account wind resistance, etc.
My Vette gets best gas mileage at about 90 mph, my Neon... probably 55 mph. I can cruise at a slower speed in 6th gear in the Vette, but there is more load (lug) on the engine, consequently the vacuum isn't quite as high.
Low geared V8 trucks out there proabably won't even shift into the OD gear at 55 mph. My Dad's Avalanche is around 1800 rpm at 80 mph if I remember correctly.