Re: Speedometer error

[PGAHOWELLS <PGAHOWELLS@domain.elided>]

Well friends

The centre of my rear wheel is 12" precisely above the ground. (I measured it)
The rolling radius on my tyre is therefore 6.284 feet. (Pi x D) (astonishing
isn't it? Roll out that tread its longer than me!!)
Thus my wheel must rotate 840.2 times in order to cover a mile that is 5280
feet long.
In top direct drive my rear axle is 3.7:1
Therefore to cover a mile in top gear needs 840.2 x 3.7 engine revs. 
That is 3108.7 revs per  mile in top.
Therefore 3108.7 revs per Minute = 1 mile per minute = 60 miles per hour. (An
auto, no o'drive)

So simple. Measure the centre of the wheel to the ground. (Not upwards because
the weight of the car flattens the tyre a bit!)
Find out what diff you have 3.7:1 or 3.45:1. 
Calculate how many rpm per mile in top, direct drive - with no overdrive.
Drive at this rpm, overdrive not selected, and your speedo should read 60 mph 
(Don't spend too long, you may hit something, and speedo accuracy will become
a topic that is highly secondary). 
If it doesn't read 60 mph it is wrong. Maybe its a bad cable, or the
instrument is not reading properly. May be the oddometer is right but not the
speed reading. More likely the tyres are not the original size, the rear axle
is not right for the speedo, the cable drive pinion is the wrong one, and so
it goes on.

So having worked that out I will check mine next.

Peter Howells