Changing diff gear ratio

[Loganator160]

Hi guys, 

I own a 98hilux. It's a Ln167, with both the g52 gearbox and 5l turboed motor recently refurbished. Cruising at 100 the engine is revving at 28-2900. It's loud. Can't get much more speed then that without it sounding like it's gonna blow. 

So I'm thinking of changing the diff gear ratio. Currently it's 3.9. If I change it to 3.4 am I going to low? Will this stress the drive train to much? 

I've got brand new 29 tyres  atm, so want to stick with them for a while yet. But might go 31 in the future. Did some quick maths in differences. 

3.9 to 3.4 gives 13% difference 

29 to 31 tyres is 6.8%

If I were to go both in the future that's 20%. Is that to much? It's fine taking off due to the turbo, I don't tow and tbh ain't going to be hardcore 4wding. More highway ks in the neat future from Adelaide to Darwin with a little off road. 

 

What are your thoughts and experiences?

Edited September 4, 2017 by Loganator160
Didn't post everything

[Manxman]

Utes r geared low, with overdrive for some economy.  My ln147r did abt 3000 for 100kph.  These motors stand 4500 +, but waste too much fuel after 3000.  U can do some calculations of power required to pull into a typical headwind and up a typical incline, with soft ground.  Match that against your power curve and you can pick rpm/kph.  If the maths is too hard experiment affordably.  Beware over heating if overgeared.  Try jackmaster oil filter for long fast cruising with turbo.