Does tyre pressure really affect MPG?
Yes, and more than most drivers realise. Under-inflated tyres create increased rolling resistance, which means the engine has to work harder to maintain speed. The tyre deforms more with each revolution, generating heat instead of forward motion. A 10 PSI drop across all four tyres can reduce fuel economy by 2–3%, costing a typical driver around £50–75 per year for a problem fixable in two minutes at a petrol station.
The rolling resistance explained
A properly inflated tyre is firm and round, making a small contact patch with the road. An under-inflated tyre squashes outwards, widening the contact patch and causing the sidewall to flex more on every revolution. That flexing takes energy, energy that comes directly from your fuel tank. At motorway speeds, rolling resistance accounts for roughly 20–30% of total fuel consumption, making tyre pressure particularly important on long runs.
Over-inflation has the opposite problem: the contact patch shrinks, which reduces grip, increases braking distances, and makes handling less predictable. Slightly over-inflated tyres do have marginally less rolling resistance, but the safety trade-off is not worth chasing a 0.5% fuel saving.
The numbers: pressure vs fuel economy
These figures are averages across a range of vehicles. Heavier cars and those with lower-profile tyres may show a larger effect. The relationship isn't perfectly linear. The first 5 PSI of under-inflation has a smaller effect than the next 5, as the tyre sidewall begins to deform more significantly at lower pressures.
Correct pressure: where to find it
Never use the maximum pressure printed on the tyre sidewall. That's the maximum the tyre can safely hold, not the recommended operating pressure. The correct pressure for your car is always found in one of two places:
Most cars have different recommended pressures for the front and rear, and sometimes a higher pressure when carrying a full load of passengers. Modern cars with TPMS (Tyre Pressure Monitoring System) will warn you when pressure drops significantly, but the system typically only triggers at 25% below the recommended pressure, which is already enough to affect fuel economy noticeably.
When and how to check pressure
- Monthly check as a minimum. Tyres naturally lose 1–2 PSI per month even without a puncture. Monthly checks keep you within the optimal range.
- Check when cold. Tyre pressure rises as the tyre heats up during driving. Check before a journey or after the car has been parked for at least two hours for an accurate reading.
- Winter adjustment. Cold weather reduces tyre pressure. Expect a loss of around 1 PSI for every 10°C temperature drop. Important to check in autumn as temperatures fall.
- Free at most petrol stations. Most UK forecourts have free or low-cost air pumps. Many supermarket forecourts offer free air. It takes two minutes per tyre.
TPMS: useful last resort, not a substitute
All new cars approved in the UK since November 2014 are required to have a Tyre Pressure Monitoring System. Direct TPMS systems use pressure sensors mounted inside each wheel; indirect systems infer pressure loss from ABS wheel-speed data. Either way, both are calibrated to warn you when pressure drops significantly below the recommended level — the threshold is typically 25% below the recommended pressure.
At 25% below recommended pressure (for a tyre inflated to 32 PSI, this means a reading of around 24 PSI), the fuel economy impact is already meaningful and tyre safety risk is real. The TPMS warning light tells you something is seriously wrong; it cannot tell you that your tyres are at the optimum pressure. Relying on TPMS as your only check means you may be running on under-inflated tyres for weeks before the warning triggers.
A quality digital tyre pressure gauge (£10–20 from any motoring retailer or online) and a monthly two-minute check before driving are the correct approach. Check cold, compare to the placard on the driver's door jamb, inflate if necessary. The TPMS is the emergency backup system, not the maintenance routine.
Some garages offer nitrogen-filled tyres which lose pressure slightly more slowly than air. The benefit is very marginal (air is already 78% nitrogen) and not worth paying a premium for. Check pressure regularly with air and you'll achieve the same result for free.