Hello,
Just doing a bit of reasearch before i start on my project. I would like to know what ratio transferbox i will need?
I currently have a 1990 200tdi 90 that i’m planning on converting to a 3.9 v8 with an r380 g/box, i’m aware of the modifications required with engine location, change of seatbox, tunnel & floor plates etc.
There are currently 35-12.5-15 on the 90 which i would like to keep. It will be used mostly on the road and very little offroad(nothing major only the occasional field).
Thanks
Mike

Like so many things. There is more than one answer. And it’s really about opinions than outright facts.

First off. You’ll probably fine regardless of which transfer box that it will run and drive better on smaller tyres. If you still want something fairly chunky see if you can get a 255/85R16. Still pretty tall but should weigh less than a 35.12.50

As for gearing. This depend what you want to achieve. If you want it peppy and to make use of the revs. Shorter gearing will be better. But this will make for a more frantic vehicle and higher rpm at cruising speeds.

A 1.6:1 would work well IMO. If you need better cruising then the additional of a Roamerdrive overdrive would help a lot.

Remember all of the tx boxes have the same low range. So your crawl speed will suffer with 35” tyres if you do plan to off road.

Which brings us round to diffs. You may want to consider running a different diff ratio either instead of or as well as.

Ashcroft has a gearing calculator on their site. Just mess about with it and get the speed ranges per Gear your are looking for.

Eg a 1.4:1 transfer box and 4.1:1 diffs might work out with more advantages than a 1.6:1 and 3.54’s. But you’d need to look at the data.

Some people with V8’s go the other way and make the gearing tall tall tall. And rely on the smooth V8 grunt. This setup might product better mpg. But is generally a lot less fun to drive.

What @300bhp/ton says ...

Depends on budget, obviously, but personally I'd plump for a change of diff/pinion gearing rather than T/box, lower in my case, so I could retain the crawl capabilities off-road. But, as you say you're mostly on-road, frankly I'd change nothing and just suck it and see how it works with what you've currently got.

The project will involve a full body off rebuild. So i would like to get it right at the build stage than have to fit it from underneath afterwards. The transfer box i have at the moment is a 1.222. I have had a quick mess about on the ashcroft ratio calculator but still non the wiser really. I’ll email ashcrofts with some questions and see what they have to say. Hopefully they will take my lt77 + 1.222 lt230 and lt85 in part ex for an r380 and lt230 with whichever ratio they advise.

1.4 t/box or 1.2 with 4.1 diffs

IMO a 1.2 will blunt performance. Good cruiser. But no fun.

The RV8 revs quite well really. I have a short geared TR7 V8 and it’s a complete riot with snappy gearing.

Not forgetting the v8 revs quite a bit higher than any of the diesels I would go for 1.4.
If you go on the ashcroft site they have a ratio calculator.

lynall said:

If you go on the ashcroft site they have a ratio calculator.

Click to expand...

Just as well it was not mentioned inpost # 2 and 4 already.

Cheers

I would say try it.
On paper mine on 33 inch tyres with a 1.2 tx and a td5 should be awful, and when the tx box was rebuilt I was going to fit a 1.4. But I do mostly motorway miles and have no issue even when towing

neilly said:

Just as well it was not mentioned inpost # 2 and 4 already.

Cheers

Click to expand...

Well done you, have a gold star