speedo and tyre wear

[helpmetoo]

ok boffins,
i presume the cars speedometer works in conjunction with the speed of revolution of the road wheels,

if this is the case and assuming the calibration is done with new tyre diameters what over read of speed would my meter be showing on say a new overall 500mm diameter wheel when worn down by 10mm of tread or 20mm of diameter?

boy racers i think kid themselves thinking of how fast they go when theyre tyres are on their last legs

presumable the over read is different for varying size of wheel ?

[kwicky]

You are really talking about the circumference of the tyre. which is Pi x D or 22/7 * 0.5(m) the distance travelled in 1 revolution.

The worn tyre would be 22/7 * 0.48(m) travelled in 1 revolution

Velocity V = d/t ....or distance travelled over time

[Hymie]

The circumference of a circle (or wheel) is proportional to the diameter (Circumference = πD).
Therefore, taking your example, should the diameter of a wheel be reduced from 500mm to 480mm � then the registered speed (based on wheel rotation) will be in error by 20 in 500, or 4%.

However, tyres are designed to provide a major component of ride smoothness � and are pressurised such there is a considerable �flat� area in contact with the road surface. Thus incorrect tyre inflation is likely to have a much greater effect on speedo error than tyre wear.

A further point to bear in mind is that car speedos typically read up to 10% high � this can be verified by driving at a constant speed (over a flat/level distance) and comparing the speed from an onboard satnav device, which have a much better speed accuracy.

[dustypuss]

speedos are connected to the gearbox drive shaft so as tyres were down the speed of the car will actually slow down even if the spedo shows thirty with good tires it will still show thirty with worn tyres but the car wont be traveling so far so will be going slower. PS Kwicky I would agree with you but my pet aardvark ate my slide rule this morning!!!!!!

[helpmetoo]

thanks for responses,

so despite speed cameras allowing say 10% + 2mph anyone who gets caught is generally much more in excess than this so cant really grizzle about their fate!