Global Fuel CalculatorWhat is the best speed for MPG?
There is no single magic number, but the pattern is the same for almost every car: economy improves as you speed up from a crawl, peaks at a moderate steady speed, then falls away quickly once air resistance takes over.
Where the sweet spot usually is
For many cars, the best economy comes somewhere around 50 to 60 mph (80 to 95 km/h) in the highest gear, on flat road with no strong wind. Below that, the engine spends more of its effort overcoming its own friction and the stop-start of low-speed driving. Above it, air resistance dominates. Gearing, engine and weight move the exact number, which is why testing your own car beats trusting a rule.
Why going faster costs so much more
Aerodynamic drag grows with the square of speed, and the power needed to beat it grows with the cube. Going from 60 to 75 mph means about 56% more drag, and from 100 to 120 km/h about 44% more. The extra power the engine must produce is bigger still: roughly 95% more for 60 to 75 mph, and about 73% more for 100 to 120 km/h.
How to test your own car
- Choose a quiet, fairly flat route you can repeat, and fill the tank to the same cut-off point each time.
- Drive one tank mostly at a steady speed, for example 90 km/h or 55 mph. Refill and enter the distance and fuel into the fuel consumption calculator.
- Repeat at a higher steady speed, for example 110 km/h or 70 mph, and compare the two L/100km results.
- Keep everything else similar: the same load, the same A/C setting and comparable weather.
Practical ways to use this
- Use cruise control on flat, open roads to hold a steady speed.
- Ease off the accelerator before a hill rather than flooring it halfway up.
- Keep tyres at the pressure on the door placard.
- Obey the limit: the legal speed matters more than the theoretical best one.
Common questions
Is driving slower always better for MPG?
No. Crawling in a low gear can be thirsty too. The gain comes from steady, moderate speeds in the highest gear, not from going as slowly as possible.
How much does speed affect fuel consumption?
Enough to notice. Many drivers see consumption rise by well over 10% going from 100 to 120 km/h. The exact figure depends on the car, so measure yours with two tanks at different speeds.
Does cruise control save fuel?
On flat, open road it usually helps by holding a steady speed. On hilly roads it can over-accelerate uphill, so a gentle manual touch can do slightly better.
Does the same apply to electric cars?
Yes, the physics is identical, and EV range falls noticeably at high speed. The kWh to MPG converter shows efficiency figures you can compare.