>Date: Sat, 22 Feb 2003 18:05:55 -0000
>From: "William Davies" <bill@rarebits4classics.co.uk>
>Subject: Re: Odometer readings on Spitfire 1500s
>
>- ----- Original Message -----
>From: Michael Hargreave Mawson <OC@46thFoot.com>
> >
> > 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
Which is why I am reluctant to take my Spit on a rallye-too much math,
trying to compensate for the inaccuracies in the speedo.
On the other hand, there is a Smith's competition speedo available now that
is quite accurate and easy to hook up. Runs off magnates like most other
competition rallye speedos and bicycle speedos. The dial is very clearly
marked so that you can easily identify the speeds. I believe it is an 80
mph speedo, but in TDS rallyes, you don't usually go much over 50. I also
believe that it is priced in the range of a new speedometer...so, if you
really want to know, with accuracy how fast you're going, that might be an
option.
Laura G.
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