my wheels are original factory fitted 15" diameter - 380mm - the tyres have an 80mm profile adding 160mm giving an overall diameter of 540mm when new. The radius therefore being 270mm. Once the tyres have 6mm of wear the radius reduces to 264mm. Approximately a 2.2% difference - this then has to be the +/- tolerence in the accuracy of my speedo.
Except if the 3 roadside mph advisory signs that I pass regularly are correctly calibrated my car speedo reads 31.5 mph when I am doing 30mph.
I have just fitted new tyres all round so even allowing for the .66 mph differential at 30mph caused by tyre wear - my speedo will still read nearly 1mph over with worn tyres
If I stick to 30 on my speedo I will probably be doig 28.5 to 29 mph - I am sure this is true due to the number of morons tailgating me with absolutely no respect for the villages I am driving through or the conditions, constantly looking down at their speedo because I am having the temerity to drive so 'slowly'.
I doubt that they have ever driven an open tractor or worked behind the cones on a motorway to even realise how fast they are going or how truly dangerous their driving is.
My best mate was killed when we were 14 by a motorist doing 35mph past a school so hats off to you mel b for taking responsibility for your own actions and ensuring you are showing due respect to our speed limits which most folk seem to have forgotten are there for a reason and not just the whim of some politician.
My mrs likes the sat nav on to beep at her if she forgets a speed limit, personally I concentrate on my driving and know what speed I am doing.
There is no excuse for speeding.