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Does car tyre pressure affect the radius of the wheel, or the speedometer or milometer (odometer) readings?
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Ron Dippold
San Diego, California, US
Yes, it does! Though not as much as speedometer fudging by your car manufacturer. Your car can only measure rotations of the axle, with expectations of tyre circumference based on its standard tyres. The distance travelled is rotations multiplied by circumference, and then, of course, the speed is the distance travelled per hour. So everything depends on that assumed circumference.
Underinflated tyres bulge out: the air goes to the sides, exactly like you pushing down on the top of a balloon. This makes the distance from axle to ground less than expected, so the 鈥渓oaded radius鈥� is smaller. Overinflated tyres will bulge less under a load, so they have a larger loaded radius than expected. A 20 per cent pressure change in a relatively soft tyre results in about a 1.5 per cent change in loaded radius. If your soft tyre suggested 38 pounds per square inch (PSI) of pressure and you had 30.4 PSI, it would have a 1.5 per cent smaller loaded radius.
When tyres are 20 per cent underinflated, your car would show 100 km travelled when you had travelled only 98.5 km
Since circumference is 2蟺r, where r is the radius, this directly translates to reading 100 kilometres per hour when you are actually doing 98.5 km/h. If you overinflated those soft tyres by 20 per cent to 45.6 PSI (which is unsafe, as 10 per cent over is about as far as you want to go), you would be doing 101.5 km/h when the speedometer is reading 100 km/h. This affects the milometer (odometer) in the same fashion. When tyres are 20 per cent underinflated, your car would show 100 km travelled when you had travelled only 98.5 km.
In reality, 20 per cent is fairly severe over or underinflation, and stiffer tyres have much less change 鈥� you would need a dangerous 30 per cent over or underinflation to get 1 per cent change in a stiff tyre. Furthermore, even a 1.5 per cent error is nothing compared with your speedometer being calibrated slightly higher by the manufacturer. By European Union law (which has been retained by the UK), the speedometer can never show lower than actual speed, but is allowed to show 10 per cent plus 4 km/h over. So you could be doing 50 km/h with the speedometer showing 59 km/h, and that would be legal, and this 10 per cent standard is typical. In practice, it鈥檚 always calibrated slightly higher, so it never reads under. For instance, when my car reads 50 km/h, radar says it鈥檚 doing 48 km/h 鈥� so my speedometer is calibrated 4 per cent over, which overwhelms any tyre inflation differences. Car manufacturers also like this because it makes your warranty expire 4 per cent sooner.
Greg Nuttgens
Porthcawl, Bridgend, UK
Yes, the tyre pressure does affect the radius of the wheel. In fact, this is how the warning system of low tyre pressure in modern cars works: a decrease in pressure results in a slightly smaller wheel radius, and this is detected by sensors that count the tyre revolutions. If the speed revolution of the car鈥檚 tyres doesn鈥檛 match, a warning light comes on.
John Davies
Lancaster, UK
A tyre isn鈥檛 a doughnut-shaped balloon! It is carefully constructed, with a 鈥渟keleton鈥� of steel and polymer wires under the rubber 鈥渇lesh鈥� so that, at the correct inflation pressure, it presents a flat surface to the road, despite that pressure being more than twice the atmospheric pressure. But that skeleton isn鈥檛 rigid, nor should it be! An overinflated tyre will bulge in the centre of the tread, as it tries to assume a doughnut shape, wearing more of this area.
This effect is small on the rolling circumference of the tyre, but it will increase so that the wheel will rotate fewer times for the same distance travelled, and the apparent speed registered by the speedometer will fall. But don鈥檛 rely on this to excuse a speeding fine!
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