GT6 Mk3 overdrive conversion axle ratio

[Roger K]

I have bought an overdrive gearbox to fit to my non-o/d late GT6 Mk3. I’m wondering whether to leave the non-o/d rear axle ratio in, to give better motorway cruising for the modern world - any thoughts?

[NonMember]

I have a vague recollection that the 3.27:1 "non-OD" diff wasn't used on late Mk3, so you've probably got the 3.89:1 diff already. What revs are you doing at 60mph in top?

That aside, whether you want to go to the hassle of swapping the diff depends on whether you feel the acceleration is too poor. If it pulls off the mark fast enough for your liking and overtakes well enough in 3rd then you'll get more benefit from the long gearing by leaving the diff alone. If you're happy with the motorway cruising as it is but really need better performance in the traffic light grand prix then change to a 3.89 (or even a 4.11 if you insist!)  My O/D Rotoflex Mk3 had been fitted with a 4.11, which was a bit short, so now I have a 3.63 (Spitfire 1500) unit which is a good compromise.

[Roger K]

Thanks for the advice.  The car's not running at the moment so I can't comment on the revs, but your comments on the single diff for later cars rings a dim, distant bell with me from the eighties when I had six very late GT6s (yes, all at once - don't ask).

I'll fit the gearbox and see how it goes.

Roger

[johny]

You can always check the serial number stamped along the underside of the diff. Its not a guarantee but should be KC for 3.28 or as Rob says might be KD....

[Colin Lindsay]

2 hours ago, Roger K said:

I have bought an overdrive gearbox to fit to my non-o/d late GT6 Mk3. I’m wondering whether to leave the non-o/d rear axle ratio in, to give better motorway cruising for the modern world - any thoughts?

A lot of discussion on that here on the forum, Roger, but I kept my original non-O/D diff when I converted and it's very relaxed. I don't see any problem with acceleration; in fact I swapped to a Spitfire 3:63 diff once and hated it - the engine revved too much and it just seemed too 'frantic', if that's the word, and durned if I could see any difference in performance. 

[dougbgt6]

I have often contemplated making the conversion and read up on it, people seem to think the non O/D diff will be OK. And really there's no need to change the diff straight off, give it a go, if you don't like it change it.

Doug

[Roger K]

I need to look at the numbers on the casing.  As Rob suggests above, I have a vague recollection that only the overdrive ratio (3.89) was fitted to the very late cars.  That said, it's not going anywhere with the rear spring out whilst I get around to fitting the correct GT6 one that arrived yesterday.  It'll be nice to get rid of the positive camber, courtesy of the Spitfire swing spring...

[68vitesse]

If this helps.

Regards

Paul

[Nigel Clark]

If your car still has the original speedo, look at the TPM or turns per mile marked on the speedo (the number on the face just below the tenths of a mile wheel on the trip odometer).

My GT6 with overdrive from new and a 3.89 diff shows 1152TPM. With a 3.27 diff the TPM figure will be lower, maybe someone here knows the exact number.

Nigel

[thescrapman]

980 for a 3.27 I think

[johny]

definitely 1000 for a 3.63 so I would expect a fair bit lower for 3.27....

[Clive]

12 minutes ago, johny said:

definitely 1000 for a 3.63 so I would expect a fair bit lower for 3.27....

Careful with that figure. On later cars (single rail gearbox?? J type?? certainly dolomites and I think big saloons as well) all speedos were 10000tpm. And the different diffs were sorted using a variety of speedo drive gears in the gearbox or OD. 

[Roger K]

Just checked the speedo, and the tpm is 980. I know for sure that this speedo was in the car in 1976 so it is almost certainly the original, so the diff should be a 3.27. Car was first reg in late ‘73. 
The number stamped on the underside of the diff is KC81416, last digit might be a 5. Any help identifying it?

[Ian Foster]

Roger

Paul WSM chart designates KC as the 3.27 (non o/d) ratio.

My Mk2 came out of the factory like that and I subsequently converted it to o/d with an o/d box harvested from a scrapyard Vitesse.

Like Colin I had a brief relationship with a 3.63 but didn't like it. In my opinion, the 3.27 is fine with a good motor in a GT6 and gives about 2750 rpm at 70mph. 

Sounds as though you already have the diff and speedo package, so I suggest you fit the o/d gearbox and see what you think.

Ian

[Clive]

KC is 3.27, from the page posted above! (looks like a kippings catalogue)

[Roger K]

Yes, it all looks good. I’ll just add the O/D in and see how I go!

Thanks all. 
What a helpful forum!