Many people think that driving at 20mph lowers fuel economy. Yet research shows that for many cars a steady 20mph is the most efficient speed if you can keep it steady. Indeed at 20mph many cars will achieve fuel consumption better than 90mpg. But what changes things is in an urban environment where you are consistently slowing down for obstacles, lights, junction and congestion and then having to speed up again.

Here we explore the engineering principle behind the energy that gets used by a motor vehicle when moving.

The physics of automobile energy consumption are pretty clear. An engine has to use energy from whatever fuel it uses for the purposes of :-

Due to moving
a) Overcoming rolling resistance
b) Overcoming transmission friction and losses
c) Overcoming air resistance

Due to climbing hills
d) Increasing the Potential Energy in the vehicle due to altitude gained

Due to increasing speed
e) Increasing the Kinetic Energy in the vehicle due to the velocity gained

Rolling resistance and transmission friction are broadly linearly proportional to to speed.
Air resistance is proportional to the square of speed.
Potential energy is linearly proportional to the height gained.
Kinetic Energy is proportional to the square of the speed gained.

a) and b) are very similar for vehicles at 20mph or 30mph.
c) is proportional to speed squared but is relatively low at 30mph or below.
d) is not affected by the speed at which you climb a hill. That only affects the rate at which you use fuel per second, the total amount required to climb a given altitude will be similar regardless of speed.
e) is the one that determines urban fuel consumption the most. Getting a vehicle to 30mph takes 2.25 times that to get to 20mph (ie 30×30 compared to 20×20). Given that in most urban setting there is repeated slowing down and often stopping for hazards, obstacles, congestion, lights, it is the repeated depletion (braking) and then acceleration to peak speed that uses most fuel. That is why a 20mph limit cycle will require less than half the energy than a 30mph limit cycle for acceleration.

Whilst the sweet spot of the engine in terms of efficency may make a slight difference this is not significant.

So, the mechanics and physics are quite clear. Smoother driving to a lower limit will always require less energy, less fuel and produce fewer emissions than repeatedly accelerating to a higher limit.