Diff ratios?

[JAGAZ]

I have my rear diff out of my 420 to replace a leaky pinion seal only to find some one had done it not long before i got the car and seem to have used the wrong puller on it and now the pinion bearings aren`t seated properly and have play in them letting the pinion move around and so the seal will not last long.In short rather than just fit new bearings and seals i now wish to change the diff for one with a better ratio to lower the revs while cruising.I believe an xjs one will do the job but wish to make sure hence the request for diff ratios,anyone know where i can find what is fitted to what .I think my 420 auto has a 3.4 to 1 but not sure,every where i look seems a bit vague on ratio info,cheers ,Howard.

[ecosselynx]

Hi Howard

First you need to check your diff regarding ratio, originally there would have been a plate on one of the back plate retaining bolts, this would have had a number on it, example 43/13 which would equal 3.31:1 ratio etc. Best to find out what you have first before trying to decide what ratio you want.
Other way to do it, is chalk the tyre and the prop shaft flange, then rotate the flange one turn, counting the turns of the wheel.
I'd suggest you give this a fair bit of thought (must admit I don't know what ratio's the 420's came with, without looking it up). You first need to ask yourself a few questions.
a) General cruising speed (motor ways), that will probably be 70 MPH.
b) Do you do a lot of A road motoring or even B roads.
c) are you looking for acceleration off the light! probably not.
d) While looking for a good cruising RPM, you still need your bottom gear to be able to pull you from stop (including a hill start).
e) You don't want to go to high in the rear end ratio that top gear is only good for motorway cruising and the rest of the time your in third or what ever.

FYI
I converted a XJ 3.31:1 crown and pinion into my Mk2. this give me about 2500 RPM at 70 MPH and can get 22 MPG loaded up (could do more on a standard motor). I can also run down to 45/50 in top (depending on road/traffic).
Feel free to get in touch should you have further questions.

Only my views and opinions

John

[JAGAZ]

Thanks John for your veiws,they posed some valid questions and gave me some food for thought.After stripping and checking the rest of the diff i found the it to be in good fettle so it will be more cost effective to just replace the outer pinion shaft bearing that was damaged by someone impacting the pinionshaft and outer bearing race into bearing cup on the casing.
I don`t do much motorway work and most of my driving is A and B roads so in reality fuel consumption wouldn`t be affected that much and then i would also have to have the speedo recalibrated too.
Thanks for your input it helped me come to a sensible conclusion.Howard.