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Does car tyre pressure affect the radius of the wheel, or the speedometer or milometer (odometer) readings? (Continued)
Sam Edge
Ringwood, Hampshire, UK
Modern road-vehicle tyres are made of extremely tough synthetic rubber and have steel wire and mesh reinforcement embedded within them, so they are far less elastic than, say, the tyres on my childhood tricycle. The composite structure makes them much harder wearing and safer, but also makes them extremely difficult to recycle.
Nonetheless, the radius of the tyre does increase with increased inflation pressure. Therefore, a speedometer that uses the rotation rate of some element of the transmission will be affected by the radius of the driving wheels. However, speedometers are generally calibrated conservatively, so it is unlikely that an over-inflated tyre, whether by mistake or because of frictional or climatic heating, is going to cause a low enough speedometer reading that one gets an inadvertent speeding infraction.
The effect can also be harnessed for diagnostics. My partner’s car measures and remembers the relative rotation rates of each wheel. Should one of them start rotating faster compared with the others, indicating its radius has decreased, probably due to loss of pressure, it flags a dashboard warning. After reinflating the tyres to the recommended front and rear pressures, she pushes a button on the dashboard to tell the system to recalibrate.
Some cars now have electronic barometers in each wheel that can measure and report the pressures directly. Military wheeled vehicles, especially amphibious ones, often have both pressure sensors and compressors on each wheel. This allows the pressure to be monitored and modified on the go to minimise drag in water and optimise traction on different types of terrain.
Danny Colyer
Bristol, UK
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It is unlikely that an over-inflated tyre is going to cause a low enough speedometer reading that one gets a speeding infraction
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For many years, my morning cycle commute was 5.37 miles (8.64 kilometres), according to a computer calibrated over a measured mile, and using a magnet attached to my 20-inch front wheel at a tyre pressure of 80 pounds per square inch (psi).
If the tyre were at 100 psi, then the computer would consistently show a ride distance of 5.36 miles. If the computer showed a ride distance of 5.39 miles, then I would know that my tyres needed pumping up. This doesn’t necessarily show that the tyre diameter varied with changes in air pressure (although it seems likely). Instead, it shows a variation in the deformation of the tyre while it is rolling.
As car tyres are less pliable than those of bikes (but also run at lower pressures and carry greater weight) and car odometers are less precise, I wouldn’t like to extrapolate these observations to car tyres.
Andrew Shead
Tulsa, Oklahoma, US
Yes, tyre pressure does affect the wheel radius, fuel efficiency and odometer readings of the vehicle. I can tell when I need to reinflate my tyres because my average speed is reduced by the higher rolling resistance of soft tyres, and the odometer reads an extra tenth of a mile.
That’s because the slightly smaller wheel radius increases the number of revolutions of the wheel that are needed to complete the entire journey, and the odometer sensor is activated by a magnet attached to the spokes. In an automobile, the reduced rolling resistance of higher tyre inflation pressures does make a significant difference to fuel consumption.
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