While the most economical driving speed changes with every vehicle, the general consensus is that it is around 50 mph (80 km/h). There are a few other things you can do to maximize efficiency, however. First, try to maintain a constant speed; this will make your engine run as efficiently as possible — use cruise control on flats if you have it. Second, drive at the highest possible gear within the speed limit; this again helps your engine run as economically as possible. Other tips include turning off AC and having as little weight in your car as you can. Read more