speedo accuracy

[drew@scirocco.cs.uoguelph.ca (Drew MacPherson)]

My $.02 CDN on the speedo issue (along the same lines as Ron's)  Both the 
Scirocco and the winter-beater Jetta have 7A transmissions in them.  The 
Scirocco has the stock 220KM/h speedo, and the tach is a W-field 
alternator driven unit from an 89 Jetta TD (ranging from 0-5000rpm)  The 
Jetta has a 16V speedo (260km/h) and tach (0-8000rpm)  The speedo drive 
gear matches the R&P of the transaxle (3.94 in the 7A)

These two cars read as dead on as I can reckon by eye for RPM vs. 
indicated speed on the speedometer - ie. 100KM on the clock is 2575 rpm on 
both tachs (admittedly harder to pinpoint on the 8K tach due to scale...) 
which impresses me, especially as the Scirocco's tach runs from the pulses 
on the alternator, not the ignition, which means it's more prone to error 
(belt slip, different pulley diameters etc)

I know that the Scirocco's actually speed will be off very slightly
(compared to the indicated speed) because it has 185/60R14, which results
in a slight increase in travelled speed (the rpm difference at 100km/h is 
less than 50 though)

Personally I'm impressed by the consistency of VW's speedos, even if they 
are optomistic (which I have not had occasion to determine.)  The fact 
that these two cars with different tachs and speedos come up with the same 
readings as can be calculated from gear ratio and tire diameter is cool.

Drew