[Vwdiesel] Tacho Find update

[slatersfb at aol.com]

I have small digital tach in my 91 n/a jetta installed by PO. Small transduser counts blades on alternator fan. It reads 3500 rpm @70 mph. I checked this digital tach against a resonant reed tach resting against shifter. Negligable difference, so it seems accurate. Hope this information helps you.
 
Bob in NY
 
 
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Further to the tacho find: Its maximum (FSD) is only 3500rpm
which I initially thought was too low BUT...
I've now wired it in and it's temporarily resting on the
drink tray by the gear shifter.
It seems my car is idling at a little under 1000rpm (maybe
950). Glowplugs switched on drops idle down to about 800rpm.
At 3000rpm (tacho) I'm travelling at about 70mph. FSD 85mph
(enough)?

Using one of those calculator spread sheets through a link
from Roger's site (A very good site  with over 30000
hits![Rogers that is 8o])
I got an answer of 70.2mph.
However I cannot work this out mathematically...
tyre diameter 23"
gear ratio for 5th I'm assuming 0.73 cant find transmission
code!!
final drive 4.11

One wheel on the ground and one wheel rotating whilst in 5th
gives 5/8ths of a rotation for 1 rotation of crank.
Is this figure halved because of the split differential
effect to 5/16ths?
Tyre circumference is about 6ft...
Combining I get 65mph @3000 which I think is wrong  :o(
Any boffins out there?
Thanks
Miser