[ajengli]

I always suspected my speedo was reading faster than I was actually going. I obtained a GPS and confirmed that it's about 10% out. i.e. when reading 100mph I'm actually doing 90mph.

 

[ajengli]

Did a trip upto Birmingham today in my Merc, the sat nav unit was indicating that the speedo was 4-5% out. (When the speedo was reading 90 I was actually doing 86).

I then started to wonder if the amount of tread left on my tyre would have any significant affect on speedo reading. Well....

My Merc has 18" alloys, fitted with 275 35 tyres.
When tyres are new they come with 7mm of tread (I think), the limit (in UK) is 1.6mm.
diameter of wheel in mm = 9*25.4 = 228.6mm
depth of tyre wall = 275 * 0.35 = 96.25mm
max wheel diameter = 228.6 + 96.25 + 7 = 331.85mm
min diameter of wheel = 228.6 + 96.25 + 1.6 = 326.45
With wheel diameter being directly proportional to speed then the difference is (331.85-326.45)/331.85 * 100 = 1.6%!!

Not much really.