High Ratio Diff/Transfer Box or Overdrive with standard diffs?

[Alan Drover]

Series Land Rovers on the standard 4.7 diffs are undergeared but what's better to raise the gearing. Overdrive has been an option for decades but recently higher ratio diffs or now transfer box ratios are now offered.
The problem is that all gear ratios are raised in high with the transfer box and both high and low ratio range with the diiff which can cause problems when towing. I recently had the experience of a 3.54 diff when the incorrect one was sent instead of a 4.7. The 2.5 coped but it was really too high geared. I'm not in a particularly hilly part of the country but there's one hilly and twisty road where a high ratio would entail far more gear changing.
Overdrive on the other hand offers a choice. The Fairey overdrive has a ratio of 0.782:1 which if my assumption is correct gives a final ratio of 3.68 (4.7x0.782).and being mechanical can be used in all gears. This is not far short of a 3.54 diff and the speedometer doesn't need changing. I did try overdrive with the 3.54 diff which gave a final drive ratio of 2.7, far too high for a 2.25 engine. The 2.5 pulled it but not very enthusiastically and I reckon a Tdi might struggle. All right for a V8 though.
My preference is overdrive with standard diffs, overdrive 3rd is particularly useful where 3rd gear is too low but direct top too high.
What do others think?

[Kevswb]

If you are towing regularly roamerdrive/overdrive all the way.
If you are not towing a good 2.5 with a high ratio box would be fine.
The best combo I have seen was a 109 200tdi with overdrive & 3.54 diffs.

HTH

[Craig T]

Overdrive would be my weapon of choice.

Never really seen the point of the Ashcroft transfer gears. You need to machine and modify the transfer case so it is a one way road then you are fitting components that are not standard Land Rover so in the future, unlikely to be made again so if you need a new one, no parts and you can't go back to standard as the transfer case is not suitable.

3.54:1 diffs are good if you have the torque to pull them along and off course that effects all gears including low ratio and messes up the speedo.

An overdrive gives a useful 5th gear or a splitter gear but I only ever used mine as a 5th for slightly more relaxed cruising. It doesn't turn it into a race car or anything, top speed will still be around 60mph simply due to the shape of a Land Rover but at least you can reduce the revs a bit.

Craig.

[Alan Drover]

I use my overdrive as a 5th gear too.

[Serious Series]

I have an Ashcroft HRT fitted but I used a spare case to be modified I still have the original transfer case and gears in case of an issue.
The reason I fitted at the time was I often did laning on my own heavily loaded going to a mountain bothy I was concerned that my fairey over drive would fail in the spline coupling , I also was trialling once a month and feared I would kill overdrive doing that.
HRT case was initially fitted when I had a 15J 2.5 deisel in my S3 I upgraded to a 200DI a year or so later to reduce the number of times I had to drop into 3rd gear on long hills.
The HRT setup makes 1st gear usable never needed to use low ratio like some do pulling a way with a trailer , regularly pull a horsebox type trailler with upto a ton of scrap metal for the Scouts funds.

[Peter Holden]

The best diff upgrade for a series with a standard engine setup is the rover 60 4.3 diff.  Quite rare these days but are usually in quite good condition.  My nephew happens to have 4, 2 have gone into an 80" with a 2.25 petrol engine fitted.  They are of a similar ratio to an engaged overdrive.

Peter

[Gritstoner]

Splashed out and fitted a Roamerdrive to ours, made a huge difference, 200tdi on 750 tyres, don't forget tyre sizes make a difference as well.

[Alan Drover]

MPH per 1,000 rpm, 4.7 diff.
6.00 tyres.
Direct top 15, o/d top 19.17.
7.50 tyres.
Direct top 16.5, o/d top 21.1.
Fairey overdrive 0.782:1 ratio.

[whitehillbilly64]

Hi Craig,
"You are fitting components that are not standard Land Rover"
I was under the belief all components were standard land rover parts?

whitehillbilly

[diffwhine]

I've looked into this and will probably do it on my Station Wagon. As far as I can tell, the intermediate gear is different is is the mainshaft output gear and the high range gear on the transfer box mainshaft. You do also as Craig says, need to re-machine the transfer box for this to work.

I'm not too bothered about putting back to standard. The original transmission I will refurbish and put aside in storage really just to maintain originality in the future. I have a transfer box casing to build up for the High Ratio conversion and I'm building a hybrid gearbox.

[Cedric]

Overdrive with 4.7 diffs in my 88 on 7.50s has been my choice for 30+ years with a BMC 2.5 diesel, Rover 2.25 diesel then petrol and now a Rover 2.5 petrol.

It has offered the most flexibility over the years. Normally running light, but towing up to 2.5 tonnes. When towing, direct 4th is the norm, with the overdrive for cruising on the flat or downhill.

I haven't really needed the splitter on third.

If I was to do more heavy towing, I'd probably change tyres to a 205x16 or equivalent.