The Energy Saving Trust says that the most efficient speed you can travel in a car in terms of achieving the best fuel economy is 55-65mph. Any faster, though, and the fuel efficiency decreases rapidly. For example, driving at 85mph uses 40% more fuel than at 70mph (oh, and it's illegal too).

The most "eco-friendly" or fuel-efficient driving speed for most modern internal combustion engine vehicles is typically between 35 mph and 50 mph (56 km/h to 80 km/h). At these speeds, the vehicle is usually in its highest gear with relatively low engine RPM, maximizing the distance traveled per gallon of fuel. Once you exceed 50 mph, aerodynamic drag becomes the dominant force, and fuel efficiency begins to drop significantly; driving at 75 mph can use up to 20% more fuel than driving at 60 mph. For electric vehicles, the "sweet spot" is often even lower, around 20 mph to 30 mph, as they don't suffer from the same mechanical idling losses as gas cars. To drive more ecologically, you should also focus on "hypermiling" techniques such as gentle acceleration, maintaining a steady pace, and anticipating traffic to avoid unnecessary braking, which wastes the kinetic energy your engine worked hard to produce.