What speed is most fuel efficient?

Every car has a range of speeds where it uses the least fuel per kilometre. Drive much slower or much faster than that, and you pay more for the same trip.

Why speed changes fuel use

Two things matter most:

• Air resistance. Drag rises quickly as speed goes up. Going a bit faster on the highway takes a lot more power to push through the air.
• Engine efficiency. Engines are least efficient at very low loads and very high revs. In top gear at a moderate speed, most engines work in their efficient range.

At very low speeds, in first and second gear, a car also uses more fuel per kilometre. That is part of why stop-and-go city traffic is expensive.

Where is the sweet spot?

For many ordinary cars, fuel economy is best somewhere around steady moderate speeds in the highest gear, and it falls off noticeably at higher highway speeds. The exact figure depends on the car’s shape, engine, gearing, tyres and load. A large SUV with a roof box will lose efficiency sooner than a small hatchback.

Rather than chasing a single number, use this rule: on fast roads, a slightly lower steady cruising speed nearly always saves fuel.

Steady beats slow

Holding a constant speed matters as much as the speed itself. Constant small speed changes waste the energy you just paid for. Look ahead, keep a good gap and let the car flow with traffic.

Electric and hybrid cars

Electric cars and hybrids recover some energy when slowing down, so stop-start city driving costs them less. At highway speeds, air resistance still matters just as much, so slower cruising extends range.

Find the sweet spot for your car

1. Add your vehicle in Road Rank so each drive shows an estimated fuel cost.
2. Drive the same highway route a few times at slightly different steady speeds.
3. Compare the cost and time for each.

You may find that a few minutes extra on a long trip saves more than you expected. For more ideas, read eco-driving tips.

Always follow the limit and conditions

Saving fuel never means driving so slowly that you hold up traffic or create danger. Drive at a safe speed for the road and conditions, within the limit.