Does a car use less or more if ?

The fast you go, the more fuel a car uses, but the majority of the cars have the optimal consume/distance rate around the speed of 70 km per hour.

If your running a constant speed then you want to have your RPM's as low as possible. But if you're driving in the city don't drive fast, that wastes your gas (like a boss). If you're driving in the city, the best thing to do is drive the speed limit, don't accelerate quickly and coast whenever possible (downhill).

most cars have a certain speed when it is more fuel efficient. a car is most economical at low revs so try to shift up to higher gears as quickly as possible. i'd say it would save the most fuel in the highest gear at low revs.