In this post, we look at what happens when your tyres are underinflated and try to get to the bottom of how much it could cost you.
Underinflated tyres and fuel costs
You’ve heard that not keeping on top of your tyre pressures decreases your fuel economy, causing more pressure on your wallet and on the environment.
The reasons are straightforward. The lower the pressure inside the tyre, the greater its ‘footprint’ — in other words, the area of tyre that’s in contact with the road. Every square centimetre of contact adds to the friction between the tyre and the road, usually referred to as rolling resistance. And as rolling resistance increases, so does the amount of force that the engine needs to generate to keep the vehicle moving.
That’s the theory, but what difference does it make in the real world? The answer depends on who you ask.
- Some experts suggest that even a modest difference in tyre pressure can make a big difference. For example, GT Radial, who carry out extensive testing, reported that when tyres are just 5 psi underinflated, fuel consumption rises by 10%. That’s a 2% increase per psi.
- However, other sources put the estimate well under that. Consumer magazine Which? tested a medium-size family car and found that reducing the tyre pressure by 15 psi only led to a 3% rise in fuel usage. That translates to 0.2% increase in consumption per psi.
Obviously, there’s a huge difference between the two estimates. Which one should we believe? We looked at two [sources]. According to TyreSafe, the agreed figures for the tyre industry are that a 6 psi drop leads to 3% increase (0.5% per psi). Hopping across the pond, the US Department of Energy reckons on 0.2% per psi.
So, will keeping on top of your tyre pressure save you money at the petrol station?
The bottom line is yes, but don’t get too excited about it. If your tyres are about 5 psi under in a small family car, we reckon you’ll save somewhere between 13p and 32p for a 100 miles of motoring.*
There are, however, other good reasons for keeping your tyres topped up.
Low tyre pressure and wear
When tyres are underinflated, they wear unevenly. Specifically, an underinflated tyre will tend to wear more rapidly on the edges. According to Tyresafe, this increased wear can be dramatic. They estimate that if tyres are underinflated by 10%, relative to the recommended level, tyres will return 10% less mileage.
Using these figures, if your tyre is designed to run at 30psi, running it at 27psi could cut around 2,000 miles off of its useful life, enough to do a Land’s End to John of Groats return trip. On a rear tyre, it could be 4,000 miles. That’s a lot of mileage to lose.
Underinflated tyres can pose a risk to drivers, passengers and other road users. There are a couple of reasons. Firstly, when tyres lose pressure, your vehicle doesn’t handle so well. Higher speed manouevres in particular can result in the car behaving unpredictably.
Secondly, underinflating tyres also greatly increase the risk of catastrophic failure. When the tyre is underinflated, the side wall flexes more than it should, and this causes the tyre to heat up. Eventually, the raised temperature affects the internal structure of the tyre, potentially resulting in a bow-out.
So it turns out that the true cost of underinflated tyres can’t be measured in completely economic terms; in fact, they are far outweighed by the potential human cost.
*based on diesel at £1.24 per litre and 47.2 mpg for a Renault Megane 1.5 dCi 110, using WhatCar’s figures.