for most cars, the most efficient speed is 50mph or 80kph, in most cars this speed should let the transmission get into top gear and have enough load on your car for the engine to be at its most efficient.
the answer from emissionguy is true but you can not drive in top gear at a to low of engine rpm there will be to much load drag on your engine.
have you ever gone up a big hill and found the more you put your gas pedal down you don't go faster there is a sweet spot where your engine rpm and load is matched to your speed. this is simmiler.
in theory that makes sense the less your rpm the less fuel you will burn. that would only be true on things designed to do that, all cars are designed to do alot of different tasks. fast and fuel efficient at the same time.
sorry the short answer 50mph or 80kph