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Vitesse Mk2 - Rear Diff 3.89 v 3.63


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My Vitesse is an overdrive model running a 3.89 diff as standard; which was originally sourced from a GT6.


I have an old rear diff (3.89) knocking about that needs rebuilding and I'm considering making it in to a 3.63 version to fit on the car.


As such:


1. Is it worth doing to begin with ??


2. What is my advantage other than (I think) lower revs at higher road speed and improved fuel economy ?? 


3. Financially, is this a cost effective method OR better to buy an off the shelf reconditioned unit ??


I accept that rebuilding diffs is not for the faint hearted !!


Any views / advice will be appreciated.


Thank you.



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My GT6 used to run overdrive with a 3.27 diff - to say it was long legged was an understatement. Made it a pig to pull away on any form of hill! I changed to a 3.63 and have been really pleased - seems a good balance of acceleration and keeping the revs lower than the factory fit 3.89 would have done.


I believe GT6 guru Cookie also runs with a 3.63 diff and D type overdrive.



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Richard, if you want a diff looked at speak to Pete at a club meeting. He is very good.

As to ratio, bear in mind the 1500 spit came with a 3.63, so a vitesse will pull it. However, it is very easy to get into the mindset that lower revs is a better thing, it really does depend on driving style and types of use.

Well worth having a look at the mintylamb gear comparison site


This is helpful as it will tell you what revs you will be at when you change gears. 


On my spitfire I am finding a 3.63 a bit too relaxed, and opens up the overall gearing a little too much. But I think a change of tyre size will drop things enough. However, my car is not exactly standard!


Hopefully see you Wednesday, will be a tad late as doing a short track session at Brands until 7ish, but will drop in on the way home for a spot of dinner.

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the Apline diff is a lot easier....


if you up the ratio     3.63/3.27 you drop revs and reduce acceleration  reduce gradabiliy 


if you lower the   3.89. /4.11  ratio you raise the revs increase performance improve gradability


as Clive says its a trade off to suit your driving style, if you get too high you have to drop a cog to go anywhere nippy



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Clive / Pete.


Many thanks for the input.


I think I will stick to the 3.89, it has looked after me fine this far but was worth considering.


See you Wednesday Clive.


All the best to you both.



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As has been said, a lot depends on your driving style, as well as of course the type of driving you do; if it includes a fair bit of motorway use, you'll notice the benefit of the 3.63.  One thing to mention is that you'll need to get your speedometer recalibrated.


Let us know what you decide to do.



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Hello Tom.


As mentioned, I'm sticking with the 3.89 which does the trick for me.


Basically with that diff, my revs are 50% of the speed (i.e. 50mph at 2.5k in 4th O/D etc etc).





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