To those that have checked their speed against GPS etc, do you keep checking as your tyres wear down?
Genuine question as I assumed the error of the speedo crept in as you wore your tyres down, ie driveshaft still spinning at x revolutions but you are travelling further, hence faster, on a new set than on a set at the min legal limit? or am I talking bobbins and the impact is too small to notice?
Good question.
Say the diam of your wheel rim plus tyre is D, the circumference is pi x D
Now say that your tread has worn down by 10mm over the life of the tyre.
The new diam is D-10, the new circumference is pi x D – pi x 10. Pi x D cancels, and the differential is pi x 10mm = 31.4mm. So for each revolution of a wheel, you’ll travel 31.4mm further on new tyres than old.
Is that significant? Depends how big your wheels are. Take a standard 215/60 R16 like on my Smax. Diam = 16″ = 406.4mm + 2 x tyre width (60% x 215) = 258mm, makes total 664.4mm. Circumference = pi x diam = 2086.2mm
So the 31.4 due to tread wear is only 1.5%, so if your speedo is accurate at saying 70mph when tyres are new, when worn down when the speedo says 70mph you’ll really only be doing 69.