Any advice on this is greatly appreciated

Iv an ex MOD 110 with a 300tdi conversion from my old disco 1. The lt77 on the 110 is mated to a 1.6.1 transfer case. With the 300tdi I get a top speed of 60ish with the engine screaming!. I looked at ashcroft for a rebuild box with different ratios but by the time you select what you want and with the deposit it was coming out at £1600 which I just don't have. I looked at second hand disco boxes, but it's a lottery as my current one doesn't leak!
I am however looking at replacing the 7x16 tyres with 285 75 16. Still don't think it's enough to make the 1.6 box gear efficiently, but don't want to start lifting the suspension really.

My question really is has anyone ran a 300tdi with 1.6 box with 285 75 16 and how did it go?

Many thanks

This all depends what you want really. There are also lots of factors at play here.

1. Are you sure it really is a 1.6? Many may have been changed long ago.
2. Where does your gearbox come from? Early LT-77's have shorter gearing than latter ones or Disco LT-77's or R380s. I say this, as an early LT-77 and a 1.4 transfer box will still scream at 60mph. Best to check exactly what you have before spending out.

Cheapest option is to buy a used transfer box, 1.2 or 1.4, but again check, as there are actually quite a lot of different ones. Ashcroft Transmissions has a handy gear calculator on their site.

I assume you mean 7.50 x 16 tyres? A 285/75 is a lot fatter (might stick out of the wheel arches). But is not really much taller, only about 1". They'll maybe feel heavier to drive, but ultimately unlikely to massively impact cruising rpm.

If you tow or haul stuff, you won't want it over geared. Also the Tdi engine is limited on power, is yours stock or tuned? Tall gearing may mean you won't be able to pull 5th gear on inclines, which may result in you spending lots of time in 4th.

Stock factory gearing isn't a bad match for the Tdi.

So from the factory a 200Tdi had the taller LT-77 gearing and works well with a 1.4 transfer box. The 300Tdi and Td5 use an R380 with essentially the same gearing and also work well. The Td5 can be made to produce a lot more power though, so they can get away going taller again.

A Discovery 200Tdi also has the taller in the LT-77, but uses a 1.2 transfer box, however a Discovery would typically be on smaller tyres than a Defender of the same year. So they actually have very similar mph/1000rpm in top. Putting big tyres on a Disco normally makes them feel over geared.

However, if you have the early short gearing LT-77, if you were to match this with a 1.2 transfer box, you'd have very similar overall gearing to the R380 (& latter LT-77) Defenders with the 1.4 transfer box. Which would probably work well in your 110. But it does hinge on which LT-77 you have.

Other options could be to look at an overdrive like the RoamerDrive, but they are ghastly expensive.

Do remember though, gains in one area are losses in other areas. With the 1.6 it probably is very flexible, letting you run below 30mph in top. Taller gearing will take this away to some extent, but obviously it'll cruise at lower rpm.

100 % it's a 1.6 ratio box. It's on an old 51A lt77. Again I belive a military suffix with shorter fist and second I have read.

It's down to its chassis just now (well has been for some time as works got in the way) but its the ideal time to do something if I can find something.

yeah apologies I ment 7.50 x 16 tyres.

It's a single cab 110, I don't tow or load anything heavy as I have a work pick up. I just don't want to strain the engine with the wrong transfer box and cause issues there.

Ok it won’t strain the engine as such.

My brother has a 1989 Ninety. It was a 2.5TD, now sporting a 200Tdi with some tweaks. He originally had the 1.4 in and much like you, is screamed at higher speeds. But it would do more like 70mph.

He now has a 1.2 from a 200Tdi Disco but retained his early LT-77 and its spot on. He has 33.10.50R15’s, which are a similar height to the 285/75’s.

I also have a 200Tdi, again modded. But it is a Disco LT-77 and also a 1.2 transfer box. Mine is much taller geared. Which you notice in 1st pulling away and 30mph is 3rd gear really unless it is level ground and best part of 50mph before you use 5th. It will sit at 80mph however. But a non tuned Tdi in a 110 might struggle on hills in top and feel a bit lethargic acceleration wise.

1.4s are cheap as everybody wants the 1.2s.