Tyre pressure is the cheapest fuel-saving lever a driver has, and the one most often ignored. DVSA MOT data and periodic AA and Michelin surveys have repeatedly found that a large minority of UK cars — often cited around one in three — are running at least one tyre below the manufacturer's recommended pressure. The usual shorthand is that under-inflation "wastes fuel", but by how much? We pulled together what published UK and European road-test data actually shows for a fairly typical shortfall of 4 PSI.
Why 4 PSI is the number that matters
Four PSI is not an arbitrary figure. It is roughly the drop you would expect over two to three months of normal use without a top-up, and it is also the threshold at which most direct TPMS systems on newer cars are legally required to warn the driver. In other words, a car can be meaningfully under-inflated and still be inside the legal warning band.
The recommended pressures on the door pillar or fuel-filler flap are set by the manufacturer for a specific tyre size, load and speed. Michelin, Continental and Bridgestone all publish rolling-resistance data showing that resistance rises broadly linearly as pressure falls below that placard figure — which is why the fuel-economy penalty scales in a fairly predictable way.
The AA and RAC have both pointed to under-inflation as a common cause of avoidable fuel spend, alongside roof boxes and heavy boot loads. It is also one of the few faults that gets steadily worse the longer it is left.
What the road-test data actually shows
Tyre-manufacturer rolling-road tests and independent European consumer-magazine road tests broadly agree on the order of magnitude. A shortfall of around 4 PSI (roughly 0.3 bar) on all four tyres tends to cost in the region of 1.5% to 2.5% in fuel economy on a mixed cycle. On motorway running the penalty is at the lower end; in stop-start urban driving, where rolling resistance is a smaller share of total losses, it is smaller still — but still measurable.
Larger shortfalls scale worse than linearly. Tests published by Michelin and by the US Department of Energy — the closest thing to a controlled reference dataset — put an 8 PSI shortfall at roughly a 3% to 4% penalty, and heavier under-inflation quickly runs into tyre-wear and safety territory long before the fuel bill becomes the main problem.
For a UK driver covering 10,000 miles a year in a car averaging 45 MPG, a 2% penalty is around 4 litres of petrol a month. At the pump prices reported in the DESNZ weekly road fuel statistics through 2024, that is broadly £5 to £6 a month, or £60 to £75 a year, for a fault that takes two minutes to fix.
Petrol, diesel and EVs behave slightly differently
The percentage penalty is similar across fuels, but the cash cost is not. Diesel cars tend to do more miles and cost more per litre, so the annual pound figure is usually higher than for an equivalent petrol. Company-car and van drivers, who typically cover the highest annual mileages in the UK fleet according to ONS travel data, therefore stand to gain the most from a monthly pressure check.
Electric vehicles are a slightly different case. Because EV drivetrains are highly efficient, rolling resistance is a larger share of total energy use, especially at motorway speeds. Under-inflation on an EV tends to show up as a noticeable range drop rather than as a fuel bill — often quoted by manufacturers in the 2% to 5% range for a similar shortfall.
Load matters too. A car running close to its rated load — full boot, roof box, four adults — is more sensitive to under-inflation than the same car with just a driver. Manufacturers publish a second, higher pressure figure for exactly this reason.
The practical routine
None of this requires any equipment beyond a decent gauge. Forecourt gauges vary in accuracy, so many drivers keep a £10 digital gauge in the glovebox and use the forecourt hose only to add air. Pressures should always be checked cold — before driving more than a couple of miles — because a warm tyre reads 2 to 4 PSI higher than a cold one and will mislead you into under-inflating.
A sensible rhythm is a check once a month and before any long trip. It is also worth checking after a sharp cold snap: tyre pressure drops roughly 1 PSI for every 5°C fall in ambient temperature, which is why so many TPMS lights come on during the first frost of the year.
The takeaway
A 4 PSI shortfall is not dramatic on any single tank, which is exactly why it goes unfixed. But on published rolling-resistance data it costs the typical UK driver something in the region of 1.5% to 2.5% in MPG, or roughly £60 to £75 a year at 2024 pump prices — plus faster tyre wear and worse wet braking. A monthly two-minute check with a cheap gauge is one of the highest-return habits available to any driver.
