Change Diff centres or Not

[JuzGU]

Hi All
I recently bought another patrol, this one is a black 2005 series IV 4.2 turbo diesel
It is running 35" muds with a 2" lift and has hg41 diff centres
The problem is that the speedo is out and i am trying to work out whether its better to just change the gear for the speedo to correct it or to change to different diff centres, maybe go up to 4.6 or 4.8 to help fuel economy also
The speedo reads 90kmh when the actual is just over 100kmh ( 12% out) and it sits on about 2200rpm at about 10 pound boost
The max boost is 22psi
I do alot of towing with this car and want to improve the fuel economy but also keep the power when towing up hills,
What diff centres would bring it back to close to standard while still running the 35"s

Thanks for any help

[nissannewby]

4.6's will bring in back pretty close to factory mate.

[the evil twin]

What he said ^^^^

Rule of thumb is for every 2 inches of tire size increment you change a diff increment so you went from 31's to 35 's which is two tyre sizes so 4.6 is two ratios up from 4.1.

I wouldn't go for the 4.8's

[JuzGU]

thanks for that guys
Now i just have to find a cheap set of 4.6s

[Craig.td42]

Hey mate I would probably be looking at a set of 4.375's. I have these ratios in my 03 GU 4.2 with 35 and the speedo is out by about 1-2km/h at 100km/h.

Are you sure that the ratios are 4.11's because it sounds like the ratios are 3.9's (which is standard ratio in this vehicle) for the speed and % that your speedo is out by. Before I changed the Ratios in my wagon I was doing close to 2000 rpm at 90k (on the speedo) which was actually 100k on the GPS.

Have you checked the Placard on the drivers side of the engine bay? (HG 39, HG 41 or HG 43.)

Cheers

Craig