On Sat, 22 Feb 2003, at around 18:05:55 local time, William Davies
<bill@rarebits4classics.co.uk> wrote:
>
>----- 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.
Hi Bill,
This is the only 1500 I have ever driven, and the gearbox was changed
within weeks of an engine strip-down/rebuild. It may be that I do have
very poor acceleration, but I just don't realise it. It is a lot
better than it was when the engine was shagged out! Alternatively, is
it physically possible that a single gearwheel has been replaced with
one from a GT6, to give a quieter/smoother cruising gear?
> 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!).
John? Are you there? Is there a speedo drive gear that could have
been fitted to a 1500 gearbox that would give a 9% *low* reading on the
correct speedo? And which speedo drive gear should I replace it with?
>
>> 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,
Ah, well, you see, I did have a problem with the original speedo in this
car, and fitted a replacement, which I then calibrated to the rev
counter (assuming a stock gearbox). The magnet *is* tired, and it
required a 7-9 mph tweak at rest to give a "correct" reading at speed.
"Correct" in this instance meaning 18 mph per 1000 rpm in fourth. The
speedo still reads 18 mph per 1000 rpm, so it hasn't deteriorated any
further over the last year.
Thanks for the continuing advice and assistance.
ATB
--
Mike
Ellie - 1963 White Herald 1200 Convertible GA125624 CV
Connie - 1968 Conifer Herald 1200 Saloon GA237511 DL
Carly - 1977 Inca Yellow Spitfire 1500 FH105671
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