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transmission speedo reduction gears and stuff



OK, this has been a burning question for quite some time:
    The rieger (FN tanny) speedo reads about 10% faster than actual speed. I figure that is due to
the gear in the tranny, no big deal really, but the rieger has 195/50's up front (yes, i know, they
dont belong on 9" wide wheels, but i wasnt the one that put em on, damn it). I would think that
195/50's ought to at least be close to any kind of silly old 13" 70/80 series that would have been
used back in the original FN tranny days, but it isnt close enough as it is still 10% off (i.e.
tires must still be bigger for it to match).
    So, on to my question: What is the exact tire size that was used with FN trannies to give a
CORRECT speedo reading??? Even better, is anyone using an older style tranny with some other
wheel/tire size that makes the speedo correct, and/or how far is it off by?
    Also, can someone give me the quick version of what the deal is with the 2 types of speedo
gears?
Thanks
Al

Allyn Malventano, ETC(SS), USN
87 Rieger GTO Scirocco 16v (daily driver, 170k, rocco #6)
86 Kamei Twin 16V Turbo Scirocco GTX ('it has begun', rocco #7)
87 Jetta 8v Wolfsburg 2dr (daily driver, 260k, 0 rattles, original clutch, driveshafts, wheels :)