Speedo/diff mis-match maths(aargh!)

[GT6boy]

After a particularly draining day dealing with the recalcitrant trailer trash at work, my feeble brain has given up. >:(

Can anyone advise what pinion gear I should have in my GT6 with J-type OD? I am not sure what ratio the OD is. The diff itself is 3.63. Am I right in thinking there are only two options 18 teeth and 20?
Also, does it matter that the speedo is from an early Spit? I'm presuming the speedo is purely mechanical and has no matching for specific diff set-ups(or is that wrong: presumption being the mother of all fu##-ups).
Currently the whole set-up reads slower than reality by a factor of 1.125- not good for speed cameras :B
I'd have stiff drink, but i'm on-call :'(

[JohnD]

The pinion needs to match the crown wheel, you can't mix'n'match.
The speedo needs to match the final drive ratio of the diff - the overdrive is irrelevant.

Sorry I can't be more helpful, but that might help your search.
John

[Richard B]

You could always get your speedo recalibrated if you cannot find a viable combination.

[GT6boy]

SpitBang wrote:

P.S. I'll have a drink on your behalf if it makes you fell any better :)

Steve: you little tinker!  ;D
So then guys, anyone know which crown wheel/diff matches to which pinion(colour and tooth combo) and how to identify the speedo's to match.

Better day at work today incidentaly. Phew....

[Richard B]

I've never found a definitive list; you have to hunt through the websites of Rimmers, Canleys and anyone else's you can think off. Comparing part numbers and part descriptions.

UK 1500 Spitfire had 3.63, USA Spitfire had 3.89 (I think), 2000 has 4.11 2.5 has 3.45.

Hope that helps?

ps I have a similar problem with a certain Herald I look after J'Type and a 4.11 diff.

[Clive]

I have never considered changing of the speedo drive, just matching speedo to diff. So thye 4.11 uses early spit speedo (in J's herald) and the 3.63 needs spit 1500 speedo. They should then be right. However no doubt there are other things to prove me wrong again.

[rotoflex]

GT6boy wrote:

After a particularly draining day dealing with the recalcitrant trailer trash at work, my feeble brain has given up. >:(

Can anyone advise what pinion gear I should have in my GT6 with J-type OD? I am not sure what ratio the OD is. The diff itself is 3.63.

The D type was the only OD sold w/GT6's originally, & its ratio in 4th gear was .8 (the transmission's output w/o OD is 1).

I dunno what the J-type's output was, it could be the same.

The GT6 Mk3 parts catalog lists only 2 diff units:
3.27 -- Normal fitment (i.e., non - OD cars) & Germany OD.
3.89 - OD cars all markets except Germany (which just kept the 3.27 diff for OD & non-OD cars).

It has been heard many times many places many folks saying though that the best diff for the GT6 OD cars is the 3.63 diff supplied w/Spitfires in the UK.  I wouldn't know, the 3.63 diffs are all but unknown in the US.

[Bob Bowling]

I had a similar problem after fitting a new diff, but by using the sat nav I found that the error is an under read of around 10%, so the sums are quite easy and I have just left it alone.

[Nick Jones]

Bruce,

You need to visit this excellent website from the two genuises Bowling & Grippo (inventors of the Megasquirt).

http://www.bgsoflex.com/speedo1.html

The TPM (turns per mile) will be written on the front of your speedo.  

There is also a way of calculating the turns per mile needed by measuring (accurately) the rolling radius of your wheels then pushing the car forward  for a certain number of complete turns while also counting (with the help of a competent assistant probably) the number of turns that the speed cable inner makes.  A fairly simple calculation is them made giving you the TPM.  You then visit the scrapyard and take the speedo with the nearest TPM from a suitable Dolomite.  Unfortunately, although this worked well in the late 80s, Dolomites are now too rare to make this viable!

My usual method was simpler.  If you have a Dolly J type gearbox and a 3.89 diff then (provided you are on 155/80 or 175/70 tyres) the speedo from a Dolly 1500  is the one you want.  If 3.63 diff then use 1850 Dolly.  If you don't want to swap the whole speedo then just the gears will usually work.  The most accurate way would be to get Speedy Cables or Richfield Instruments to do the job but they will want the TPM measurements as described above.........

http://www.speedograph-richfield.com/
http://www.speedycables.com/

This is a useful article too
http://www.westnet.com/~mfrank/Speedometer/Smiths.html

Confused....?  Thought so!  I was very glad my new Toyota gearbox truned out to have identical speedo gearing to my old 1850/J-type hybrid!

Cheers
Nick

[GT6boy]

Cheers guys! Crikey Nick- splendid effort there. I will attempt to trawl through this and achieve resolution. It peeves me that the Speedo reads out, I can (and have) worked out the readings, to avoid unnescessary speeding, but the needle is super smooth in operation, so its a shame it reads wrong!  :B