MK2 Axle Ratio's

My MK2 started life as a 240 Auto , far as I know the axle is 4.27 .
Going by the satnav , at 60mph revs are 2.500 , at 70mph 2.700 .

Going by the manual a 3.77 Axle Ratio as fitted to 3.4/3.8 cars

60mph would be 2.300 and 70mph 2.670 revs

I was going to refurbish a 3.77 Axle and fit that , thinking the revs would come down more .
Gearbox is a all synchromesh , tyres are 215/70-15

I take it the rev counter is about right , it ticks over at around 700 smooth , it is a XJ6 3.4 engine with a generator bolted on the back of the cam , like the MK2

What are your thoughts ?

Would be nice to know other MK2’s , rev’s to mph , at 60 and 70 mph , going by a satnav not the speedo !

Is it still a 2.4 motor? If it is then obviously acceleration will be stunted.
I blew the diff (3.54) in my 3.8 ‘S’ many years ago and replaced the C/w and pinion with a 3.07 and I had also changed out the DG box with an all synchro box. Brilliant cruising revs at 60mph, from memory it was about 1,800rpm (pre sat Nav) but certainly a struggle on a hill to start if it was getting steep.

Is it still a 2.4 motor?

Well here is what I got today in my 3.8S, manual with overdrive and the stock 3.77 final drive. I currently run 195-70 X15 tires. The Garmen device shows the speedo to be dead on. At 60 MPH the tach reads 2400 and at 65 MPH 2600.

Oops missed that part :slight_smile: scim reading 101

I don’t have my factory workshop manual with me at the moment, but my E type manual tells the road speed for engine rpms and different differential ratios up the front of the book. Perhaps the Mk2 manual has a similar page - if you have the book? My Mk2 runs about 3000rpm at 70MPH. It has a DG box and a 3.54 arse end. FWIW. Paul

Hi Paul , yes I do have the manual ,

not for a 240 but .

2.4 with axle ratio of 4.55 all synchromesh box with overdrive is 3.54 ratio , that’s 3.221 revs at 70mph

looks like 3.4 3.8 no overdrive box has ratio of 3.54 , top gear is 3.54 ratio , that’s 3.221 at 70mph

3.4 3.8 with overdrive is 3.77 , in overdrive its 2.933 , at 70mph that’s 2.670

Going by the book my 70mph at 2.700 revs , is good , I did look at removing the axle cover to see what ratio my axle is , but looks to be a pig of a job , as it’s so close to the fuel tank !

You could always jack it up, put in neutral, turn the road wheel and count the rotations for one full turn of the tailshaft. Count more for greater accuracy! I did this with the good lady wife’s XJ6 - a 3.07, just to be sure. There are good tricks for this on YouTube IIRC. Tony suggested this to me some time back. Paul

Thanks for that Paul , just did this , worked out 1 wheel turn to 4.25 prop !
I am thinking the right way ?
At the moment it works out about 2670 revs at 70mph
If I fit the 3.77 axle , the revs should come down to say 2500 at 70mph !

Hi Ian, does 4.25:1 sound right? I did about 5 turns to improve accuracy. IIRC I calculated about 3.055 - which was near enough to 3.07. I don’t know what diffs are available past 3.77 as my cars have the bigger sixes. Paul

Yes sounds about right
2.4 =4.27:1
2.4=4.55:1 overdrive
3.4/3.8 =3.54:1
3.4/3.8:=3.77:1 overdrive
From my readings it looks like the 4.25:1 and the 3.77:1 with overdrive are the same in overdrive about 2700 at 70mph .
If that is the case what is the advantages in me fitting a 3.77:1 ?

So if you have a 4.27 and change out for a 3.77 you will get a reduction as below:

3.77/4.27=0.882. A further reduction of about 12% in any gear.

If you have a 3.4L with effectively a 3.77 final drive (by including the “overdrive effect” on a 4.27) I still think you’re turning too many revs. My 3.4L is the original turning a 3.54 through my DG250. It’s turning less revolutions than your car and still pulls in top gear like a steam train.The DG locks up in third gear at 1:1 (like a normal 4th gear). I think you could come down to the 3.77 without dramas. This would come in well under my 3.54 with your overdrive. Paul

This is why I would like to know what other MK2 owners are getting , at 70 mph by the satnav ,
By your calculations with a 3.77 axle at 70, revs would go down to under 2400 at 70
Manual says they should be around 2700 !

My Mk2 runs about 3000rpm at 70MPH.

My car is running less then yours by what you say , mine is 2700 at 70 !

Something not quite right?

Maybe we should both try another tachometer. Like the high range on my DwellTach - which runs to 6000rpm. What reduction do you get from the OD? I had one in Dolomite Sprint - a J type Laycock - reduction was 0.78.

Ian. 4.25 with o/d =3.748 I assmue thats what you are alluding to.

Look’s like overdrive is .778 ratio

Would the .778 be taken off the 4.27 to make 3.492 ratio ?

Both my MK2 and Stype were manual with overdrive and had a 3.54 ratio axle.
These cars were designed for UK use in the 1960s before motorways were built. Over here it can be quite hilly and that was also a consideration along with the fact you might wish to tow a caravan up one of those hills.
On the flat, you could often set off in 2nd gear and especially in the MK2 with a crash 1st, I often did.
As you have a 240, you will have the advantage of a full syncro box, so you could easily drop a couple of ratios, even down to 3.31. You might have to use 1st a bit more but that won’t be such a chore with the box you have.

So a 0.778 overdrive - same as my long departed Sprint. 0.778×4.27= 3.32. That’s a big reduction and underneath my 3.54. Now it makes sense! Some of our assumptions above were wrong. Jaguar has a standard ratio at 3.31 and it went into the bigger six cylinder cars with 15 inch rims. I think you’re on the money. But as Wardell points out first gear would really only be needed for stump pulling with your 3.4 engine. Our XJ6 has a 3.07 and it goes really well. Paul