Tyre pressure: how much more fuel you use with under-inflated tyres

The short answer

• According to fueleconomy.gov, fuel economy drops by 0.2% for every 1 psi drop in the average pressure of all four tyres: about 3% per bar.
• The ADAC puts it at 2% more fuel with 0.4 bar too little, and Germany’s Federal Environment Agency (UBA) at around 5% with 0.5 bar too little.
• The ADAC adds that 0.4 bar too little cuts tyre life by around 30%. Low pressure costs you twice: in fuel and in tyres.
• The effect is bigger on country roads than on the motorway, because at lower speeds rolling resistance makes up more of the fuel used.

Why a soft tyre uses more fuel

An under-inflated tyre flexes more as it rolls. That constant flexing turns into heat, and the energy comes from the engine: this is rolling resistance. The softer the tyre, the more the tread squashes against the road and the more it resists.

How much you notice it depends on how much rolling resistance contributes to total consumption. According to Gasolímetro’s physical model, in a car at a steady speed rolling resistance accounts for around 27% of the energy at 70 km/h (43 mph) and 19% at 120 km/h (75 mph), because at higher speeds air resistance dominates. So the same rise in rolling resistance increases consumption more on a country road than on the motorway. In an SUV, the share is similar (29% at 70 km/h).

Weight works the same way: rolling resistance grows with mass, so a loaded car with soft tyres gets both penalties. That’s why manufacturers give a higher pressure for a full load.

What the measurements show

The figures vary quite a lot between sources, because they depend on the tyre, the car, the speed and how much pressure is missing:

• fueleconomy.gov (US Department of Energy and EPA): 0.2% lower fuel economy for every 1 psi drop in the average pressure of all four tyres. Keeping them at the right pressure improves fuel economy by 0.6% on average and up to 3% in some cases.
• ADAC: about 0.4 bar below the recommended value raises fuel consumption by 2% and cuts tyre life by around 30%.
• Umweltbundesamt (UBA): 0.5 bar too little raises consumption by around 5%.
• ORNL: in track and dynamometer tests with a compact saloon and an SUV, low tyre pressure caused anything from a negligible penalty to a 10% loss in fuel economy, depending on the case and the speed.

The takeaway is a range, not an exact figure: a few tenths of a bar mean 1–2%, and a clearly soft tyre can reach 5% or more.

What it costs you a year

We take the same hatchback as in our petrol or diesel and motorway speed guides: 6.0 L/100 km WLTP for petrol (7.2 L/100 km in real-world driving) or 4.8 L/100 km for diesel (5.6 real-world), and 15,000 km a year.

It’s not a fortune, but avoiding it costs nothing. Add tyre wear (a tyre that lasts 30% less, according to the ADAC) and checking your tyre pressure is one of the best-value ways to save.

How many cars run on soft tyres

More than you’d think. In a nationally representative US sample, the NHTSA (the US road safety agency) found that 23.1% of 2004–2007 cars without a tyre pressure monitoring system (TPMS) had at least one tyre 25% or more below its recommended cold pressure. With TPMS, it was 11.8%, and among 2008–2011 models with TPMS, 5.7%.

In the European Union, Regulation (EC) No 661/2009 requires cars to have a tyre pressure monitoring system “in the interests of optimum fuel consumption and road safety”: on new models from November 2012 and on all new cars from November 2014. But the system warns you once the loss is already significant; it’s no substitute for checking the pressure.

Tips

• Check the pressure once a month and before a long trip, always with the tyres cold (before driving more than a few kilometres).
• Use the manufacturer’s pressure, usually shown on the driver’s door frame, inside the fuel flap or in the handbook. If the car is loaded, use the full-load value.
• Don’t rely on the warning light alone: it comes on late, and a few tenths of a bar already show up in your fuel bill.
• When you replace your tyres, look at the EU tyre label: according to the UBA, the best and worst efficiency classes are up to 7.5% apart in fuel consumption.
• To estimate the cost of a specific trip, use the calculator; the methodology explains where each factor comes from.

Frequently asked questions