----- Original Message -----
From: Michael Hargreave Mawson <[EMAIL PROTECTED]>
>
> Was there ever such a thing as 3.31:1 gearbox? If so, I may have one
> fitted to my car! I note that GT6s had a 3.27:1 overall ratio, which
> would give very similar results - could I possibly have a GT6 gearbox
> fitted to this Spitfire? Are they (or their internals) interchangeable
> with Spitfire ones? Can anyone come up with any other rational
> explanation why an odometer, apparently in perfect working order, reads
> 8.9% low?
Hi Mike,
The overall drive ratio is a function of the final drive, ie, the differential
assembly (though the differential component within actually has no bearing on the
drive ratio, I digress). All of "our" gearboxes give a direct 1:1 drive ratio in 4th
gear, with overdrive obviously giving a step down. The 3.27:1 drive ratio of the
non-overdrive GT6 is derived from the final drive which is geared to that exact ratio.
If you had one of these you would know it as you would have very poor acceleration
from a 1500. There are different plastic speedo drive gears fitted to various
gearboxes and these have a bearing on the speedo readings, but I don't know the
specifics. You might want to ask John Kipping about this as he's been helpful in the
past on this subject (and many others!).
> If I do have a freaky drive ratio, it might explain why the speedo in my
> wife's modern Mazda reckons we are doing 92 when travelling in convoy,
> with my revs at 4600, which would equate to 83 in a normal UK-spec
> Spit...
The usual weak link is the speedo unit itself. They were never particularly accurate
and as the internal magnets weaken with age they only become less accurate. Unless you
find something which has obviouly been swapped, it's a fairly safe bet that your
speedo is at fault,
Cheers,
Bill.
--
Rarebits4classics
.......just what you've been looking for
PO Box 1232
Calne
Wiltshire
SN11 8WA
United Kingdom
http://www.rarebits4classics.co.uk
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