Fuel consumption at 100, 110, 120 and 130 km/h: how much you save by easing off

The short answer

• From 120 to 100 km/h: 16% less fuel according to Gasolímetro’s model (the ADAC says around 15%).
• From 130 to 120 km/h: 9% less. From 120 to 110 km/h: 9% less.
• You pay in time: every extra hour behind the wheel at 100 instead of 120 km/h saves you about €14 in fuel.
• The taller and wider the vehicle, the more litres it saves by slowing down, because air resistance makes up more of its consumption.

Why going faster uses more fuel

A car cruising at a steady speed spends energy on three things: tyre rolling resistance, which barely changes with speed; the engine’s internal losses, which per kilometre fall the faster you go; and air resistance, which grows with the square of speed.

That’s why it’s wrong to say fuel consumption grows with the square of speed. Only air resistance does. If all of it did, going from 100 to 120 km/h would mean 44% more fuel; measurements show far less, around 15–20%. Even so, on the motorway air resistance is already the biggest factor, and every extra km/h costs more than the last.

For the same reason, anything that spoils the aerodynamics hurts more at speed: a roof box, open windows or a tall vehicle such as a campervan or motorhome.

What the measurements show

Oak Ridge National Laboratory (ORNL) tested 74 vehicles on a dynamometer at steady speeds between 50 and 80 mph: cars, estates, SUVs, pickups and MPVs from 2003 to 2012. On average, fuel economy fell 36% from 50 to 80 mph, and at 80 mph it was 27% lower than at 60 mph, which means 37% more fuel used. According to the US Department of Energy, the most efficient speed for cars and light trucks is 40–50 mph (64–80 km/h).

The ADAC, Germany’s motoring club, reaches a similar figure for Europe: averaging 100 km/h instead of 120 saves around 15% fuel. In its example, a petrol car uses about 6.7 L/100 km at 100 km/h and 10.1 L/100 km at 160 km/h, almost 50% more.

According to fueleconomy.gov, driving 5–10 mph slower on the motorway saves 7–14%, and aggressive driving (hard acceleration and braking) can raise consumption by 15–30% at those speeds.

How much you save and how much time you lose

We take the same petrol hatchback as in our petrol or diesel guide: 6.0 L/100 km WLTP, or about 7.9 L/100 km in real-world motorway driving at 110 km/h according to the calculator.

Put another way, slowing from 120 to 110 km/h “pays” you about €17 for every extra hour at the wheel, and from 120 to 100, about €14. On a 500 km (310-mile) motorway trip, 110 instead of 120 km/h saves about €6 and adds about 23 minutes.

In a compact SUV, the model gives a similar saving in percentage terms (17% from 120 to 100 km/h), but more litres, because it starts from higher consumption. Bear in mind that the limit on Spanish motorways is 120 km/h for cars; in the UK it’s 70 mph (113 km/h). If you’re weighing up a toll motorway against a free road, see toll motorway or free road.

Tips

• Use cruise control on the motorway: holding a steady speed saves fuel in most cases (fueleconomy.gov).
• If you’re in a hurry, slowing from 120 to just 110 km/h already saves 9%, and costs barely 5 minutes per 100 km.
• The more weight you carry or the worse your aerodynamics (roof box, bikes on the roof, windows down), the more easing off pays.
• Check your tyre pressure too: a few tenths of a bar too little raise consumption by 1–2%.
• Avoid hard acceleration and sudden braking: according to fueleconomy.gov, aggressive driving can raise motorway consumption by 15–30%.
• To estimate the cost of a specific trip, use the calculator: it works out the route and splits the distance between town, country roads and motorway.

Frequently asked questions