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4.37:1 or 4.1:1 diffs on series III SWB


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Hi List. Long time lurker here.

I have just got hold of a 1983 SWB Series III petrol 2.25 which I plan to use as a daily drive, and for  occasional touring, holidays & greenlaning. 

I see that it is common to put in Range-Rover diffs as it makes motorway driving easier, but that this destroys the low-range 1st gear creep.

The vehicle sits on 32" tyres, and is having an overdrive & some engine power upgrades, but reading the Ashcroft ratio calculator, I see that 70 mph will still mean ~3200 rpm, which on a long journey will still be deafening ...

So, rather than putting in 3.54:1 RR diffs,  I was thinking of changing to 4.37:1 or even 4.1:1 (Ashcroft supply both) .

What do you think? 

Is low range 1st usable with 4.1:1 diffs?

Will a change from 4.7:1 to 4.37:1 even be noticeable..? 

The crown-wheel & pinions supplied by Ashcroft have fewer teeth than the 4.7:1 set - but is it correct that the increased noise from these will be more than offset by the reduced engine revs ...? 


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If you’re already installing an overdrive, I think you’d be best off towait until you have that to see where you are.  A 2.25 isn’t particularly powerful; it’ll pull the overdrive and standard transmission, but not a great deal more.

I have a 109 with a Tdi using a fundamentally standard transmission and overdrive.  I did try 3.54 diffs, and even with the overdrive disengaged, it was truly awful to drive except once (eventually) up to speed on the motorway.  For a Tdi or V8, 4.1 diffs are a compromise that may work, giving a top gear ratio similar to a Tdi Defender - those engines pull the overdrive quite easily and run out of gears with 4.71 diffs, but I’m not sure a 2.25 would cope unless those power mods are significant.

You can fit the low range and intermediate cluster from the SII suffix C transfer box which will drop low range by roughly the same amount that 4.1 diffs raise it.  They and overdrive have no effect on speedo accuracy,  but the diff ratio change will.  I have done the low range swap and it’s pretty simple, though three hands are beneficial if you do it without removing the transfer box from the vehicle.

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I will definitely wait for the overdrive before undertaking open-heart surgery.

So if I understand correctly, you're suggesting that the loss of low-end grunt caused by changing from 4.7:1 to 4.1:1 diffs could be counterbalanced by reducing the low-end gearing.

I have a (vague) hunch that this would mean that the gear-spacing would increase across the rev range.

That sounds like a good solution - thanks Snagger!

(I am still curious/hopeful that the 4.37:1 diffs would be a small enough change that the SII LR gear correction wouldn't be needed...)

Edited by SmegHead
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You can see the details here: http://www.nickslandrover.co.uk/new-lows/

It doesn’t affect low gears as in 1st and 2nd - it drops all gears in low range because it’s dropping low range itself.  It means that in high range, all your gears (including overdrive) will be increased by around 17% by the diffs, but low range will have the same overall ratios as you have now.


The 4.37 diffs would probably work well with the 2.25, and if you did the transfer box mod, you’ll end up with the low range still being lower than it is with the current gears and diffs.

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  • 6 months later...

I have 4.3 diffs from Rover P4s in my Series 2 SWB, with a Fairy Overdrive too, and that's using a 2.25 diesel! You get used to driving it as a 6 speed gearbox using overdrive on both third and top. I put the 4.3 diffs in when I had 4 long journeys to make on the road each week.

I though it would improve fuel economy, but instead it's worse as I just went faster! Fish Hill, near Broadway, was a third gear no overdrive climb. The Fairey overdrive is quite noisy in a whiney sort of way.

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13 hours ago, Chris Hall said:


I though it would improve fuel economy, but instead it's worse as I just went faster! Fish Hill, near Broadway, was a third gear no overdrive climb. The Fairey overdrive is quite noisy in a whiney sort of way.

I had a similar experience with the 3.54s and overdrive with the Tdi.  The problem is that you have to use the lower gears much more, and they are lower than the standard 4th, plus 1-3 and the overdrive all use meshing gears, lay shafts and lots of bearings, so have more losses than fourth, which runs straight through the box.  Off the top of my head, a 3.8:1 diff gear set should give a similar final drive ratio as standard with overdrive engaged, but should give mildly but measureably better performance and economy because you’d not be turning any extra components and stirring any more oil.

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