Does having 8mm of tread vs 3mm of tread affect the indicated speed much? I was trying to work it out but then I got into speed expansion and other variables. Perhaps someone with satelite navigation can comment about indicated speeds and tyre wear and how it affects road speed.

Thanks for any ideas,

Ian

by my reckoning a 5mm difference in tread depth is 0.8% in circumference so it should make chuff all difference.

That's based on the tyre size on my car.

Edited 6 May, 201016 yr by trundlenut

I get 1.4%, but we're in the same ballpark.

Tyres in the form a/bxc where

a is width in mm

b is the height expressed as a percentage of width

c is the wheel size in inches

diameter of wheel and tyre (mm) = (c*25.4)+(2*(a*(b/100)))

example 195/65R15

diameter (new) = (15*25.4)+(2*(195*0.65))=635mm

Worst case - 10mm tread down to 2mm. Loss of diameter = 2*8mm =16mm so diameter (worn) = 619mm

Ratio is 619mm/635mm = 0.9748

distance travelled is proportional to diameter.

measured speed is proportional to wheel rpm

Assuming no other speedo errors, the speedo would be reading 1/0.9748 actual roadspeed.

That is 2.6% higher than actual

Edited 6 May, 201016 yr by rwbaldwin

Yes it will affect it, but it's all within the tollerence.

This is one of the reasons a speedo reads a small amount under on a new car. To allow for wear.