Rear axle ratio

[Barry153]

Hello all,
I have a 3.8 Mk2 fitted with a 5 speed Getrag gearbox and a standard 3.77 back axle.
Everything is fine but I find 1st and 2nd gears pretty low and nothing really happens until you get into 3rd. also like many people i would welcome an improvement in fuel consumption!
My question is, if I change to a 3.54 back axle will I find a worthwhile improvement for the expense?
At present in 5th gear she runs at 2750 rpm at 70 mph
With the 3.54 axle it would be 2580 rpm at 70 mph.
Would 170 rpm make it more pleasant and would it influence the fuel economy?
I would welcome opinions.
Thanks,
Barry 1530

[Martec]

Hi Barry,

Our 1961 3.8 MkII originally only had a 4 speed moss box so has the higher rear axle ratio of 3.54. I replaced the knackered moss box with the later all synchromesh box with overdrive. With this arrangement first gear is low-ish to get you moving, but 2nd gear is the acceleration gear.

I have recently done 1600 miles on our Scottish tour, mainly on back roads and over the passes (3rd gear most of the time) and averaged 20mpg. The engine is in very good rebuilt condition but runs a different ignition system and maps. Unfortunately we had to run on petrol most of the time.

Hope that is of help,

Brian

[ecosselynx]

Hi Barry

As Brian said changing to a 3.54 axle ratio will help. I run a 3.31 ratio axle with my 5 speed and find it fine reading 2500 RPM at just a tad over 70 and can still run in 5th at 50/55, which is about where modern cars change into top.

To do this change you need to get a crown and pinion gear out of an old XJ axle and swap them for the 3.77 in your axle. Three of the four bearing are same, while if I remember the front pinion bearing is different.

Anyone interested in doing this would be rebuilding the complete axle with new bearings and to do this you need the following bearings.
Half shaft, Timken 14276 quantity 2
Crown wheel, Timken 25523 quantity 2
Pinion inner, Timken 3152 quantity 1
Pinion outer, Timken 02872 quantity 1

You can of course fit higher ratios, but you 5th becomes a motorway only gear and do your overall consumption no good.

regards

[Barry153]

Thank you both for your comments, they are very helpful.
John, two questions,
Does the 3.31 ratio make first and second more usable? I find at present I am changing up the first two gears almost straight away!
Do all XK axles have the 3.31 ratio? and are they easily interchangeable for the Mk2 axle?
Thank you again,
Barry.

[ecosselynx]

Hi Barry

In answer to your questions: Yes 1st and 2nd are usable. No not all XJ's have 3.31 axles, can't remember which ones at the moment and I'm traveling. I'll see if I can dig up the ones to look out for.

[maurie]

Hi, I have a 3.8 with 5 speed getrag, just had fitted a 2.88 by Hardy Engineering , can now use 1st gear , and does 32+ mph in 5th. Had the speedo recalibrated be Speedy Cables, still goes like the clappers.
Went to Eastern Day and stayed for a week touring , did 658 miles and used
112 ltres of super unleaded . 25 mpg.not to bad. 3.77 with overdrive gives overall ratio of 2.93. Getrag 4th gear is direct drive ie 2.88, so 5th gear is
like overdrive. 0.8. = 2.30 overall ,lovely and relaxing, is not quite so frantic sounding on the motorway. The 2.88 was fitted to some 5.3 XJs etc.


Have fun
Maurie

[Barry153]

Thanks Maurie,
I have been away hence the delay in replying.
After some head searching I have decided the 3.31 is probably the best way to go for me.
The quest is now on to find a suitable diff! I have been offered an E Type 4.2 power lok 3.31 ratio that I am assured will be suitable for my Mk2 but it is more expensive than I want to pay!
Can anyone confirm that the E Type 4.2 diff will fit?
Can anyone help with finding any other suitable diff?
Grateful for all help,
Barry.

[alec2824]

I have a couple of rear axles reported to come from Mark 2's. I do not know the axle ratios though! I will have a look-see and try to establish what they are. I was keeping them for the half shafts!

[Barry153]

Hello Alec,
If it is of any help, I believe that on the ratio I require the crown wheel has 43 teeth and the pinion has 13.
Regards,
Barry.

[ecosselynx]

Hi Alec

There were not any standard Mk2 axle ratio's at 3.31.

Barry

As your increasing the ratio, your probably not going to be "burning it off the light" so paying the extra for a power-loc would be a waste of money. I don't have any books here with me at the moment, but from memory series 2 XJ come to mind.

[alec2824]

Barry 153, I am not sure how to check the ratio but what I have done is to turn the wheel end of the axle and count the revolutions of the prop shaft flange. I get four and a quarter turns of the prop shaft flange to one revolution of the wheel. Both wheel ends turned the same amount! The other axle is giving some grief in that both wheels are turning different amounts!

[Barry153]

Hi Alec,
I think that your experiences mean that one diff is a limited slip diff and the other is not.
But as Ecosse Lynx explains the Mk2 was never fitted with a 3.31 axle ratio so I don't think either of yours will be a 3.31 ratio.
Thanks for your interest.
Regards,
Barry.