So, I have a D90 with a regular Fender Transfer box. I hear that putting a disco one in as a replacement improves the drive at higher speeds which is of interest to me as I spend too much of my life on motorways and I really could do with extra MPG. Does this have to be a TD5 transfer box? Or will any old Disco transfer box do?
Thanks

Depends on your use and tyre size i had a disco box and large tyres yes motorway cruising is good but lacking a bit low end. just fitted a new standard defender transfer with large tyres and would say its perfect.

so large tyres stick to the defender box
small tyres disco tbox would be a good swap

regarding mpg going steady on the disco box on long runs 50 / 60 mph i would get good mpg 30 + ish and around town low 20's

with the standard box and large tyres it seems to be a bit more constant mpg 28 ish rough maths all round

A Disco box uses the 1.2:1 ratio rather than the 1.4:1

This will up your gearing. If you are on standard 32" tall Defender tyres, then you will be over geared. A Disco on 29" tall tyres will pull approx the same mph per 1000rpm in top as a stock Defender does.

Swapping in the taller gearing in theory lowers your cruising rpms. But the downside is, it'll blunt performance/acceleration too. And you may find a stock Defender doesn't like pulling the lower rpm at high speed. Which means you may find you have to change down more often for hills and that you might end up with your foot welded to the floor to keep at a speed it used to manage fine.

So while in theory it could improve mpg, there is also a chance you'll drive it harder and end up using more fuel.

Example being, if you normally cruise at 60-65mph, then the 1.2:1 transfer box will easily allow this at lower rpm. So should be better on fuel, but will still blunt acceleration.

If you want to sit a 80mph, then you may find a stock diesel Defender won't like this, so you'll drive flat out all the time.

Understood, i will look more into it. I think after its remap, it'll pull better, as after 100k its showing its age and I don't feel like it is what it could be which disappoints me.

I have a 90 with 300tdi with the discovery transfer box , its the auto (slightly higher output) version of the 300tdi , and its still using the auto gearbox I have 265/75 BFG MT tyres . It is good on the motorway at 60/65 but its no good for towing as it needs to be travlling 55+ to get the TCLU , which is hard to achieve on normal main roads.
If you are running a 90 on 235/75 or 750r16 then might not be a good move If you went to discovery sized tyre
with the 1.2 transfer box then you would get the same gearing as a discovery which is slightly higher than a defender.

As Richard said above disco tfer box with big tyres is a nightmare especially pulling away up a steep hill, once had to drop it into low box!
33 inch tyres std def tfer box about right still a tadge to high but liveable.
Theres loads of threads on the same subject and if you read between the lines on lots of them 5th becomes an overdrive and you end up with a 4 speed gearbox! tuned td5 might be okay but 200 and 300 find it hard going.

My 90 is fitted with a Disco box. It's just about ok with 265/75 tyres but I'm positive that it won't be too happy with larger tyres. I would love to fit a set of 33" but I don't think I should take the risk. Has anyone here got a 90 with standard defender transfer case, fitted with 35" tyres? I guess the gearing should be similar to 1.2 transfer case with 33" or am I wrong?

Hi Reiny

I have 255 x 85 Tyres which i think are 34" inch tall ran them on both a 1.2 and 1.4 box and on the 1.2 is was ok just ok. now back on the 1.4 box and its much more drivable pick up is better not driving the car half as hard as i used to.

Ashcrofts website has a Ratio Calculator enter the tyre size transfers box ratio and you will see the rpm / mph read out makes it easy to compare

http://www.ashcroft-transmissions.co.uk/calc/ratio_calc.html

My 3 door is on 35's with a 1.4 box and itll quite merrily cruise down the motorway at 70+
my 110 csw is on 33's with the 1.2 box and it was fine (until the engine problem caused issue) but previous to this it would similary do 70+ quite comfortably with no issues of towing. Even 3 tonne on the back, pulling away was no problem up hills etc.

I equally looked into this as well, I found the cost involved and benefits gained where just not worth it. After the map I can happily cruise at 70, put your foot down it still goes got up to about 90 and it still felt like it could go more. That said I spend 90% of my time on the Mway at 60 - 65, no real need to go any faster and MPG is much improved.