Thread: How to reduce drive ratio?

Dave changing the transfer box ratios sounds like the cheaper option, but it will not improve your low range performance as all low ratios are the same in the LT230 box. Also given you have a D90 you will likely have 1.41:1 ratios already, your main option is therefore to change the CW&P in both diffs, an expensive option.

You will also have to change the speedo ratios to match your diff and tyre size.

Originally Posted by Lotz-A-Landies

Dave changing the transfer box ratios sounds like the cheaper option, but it will not improve your low range performance as all low ratios are the same in the LT230 box. Also given you have a D90 you will likely have 1.41:1 ratios already, your main option is therefore to change the CW&P in both diffs, an expensive option.

You will also have to change the speedo ratios to match your diff and tyre size.

Actually, factory V8 90's have 1.22:1 ratios.

Originally Posted by cmurray

Actually, factory V8 90's have 1.22:1 ratios.

It came out of the factory as a 2.5na diesel. I will have a look on the trans case to get a number and find out if it has the original 90 one or got the rrc one at the same time as the engine swap. I think it is the original 90 and going off the Ashcroft site that would make it 1.41.

So looking here: Crown Wheel & Pinions
The 35s will have given a 21% increase in final ratio. If I were to put in 4.75 R&Ps that would give a net reduction of 12% compared to factory. Seeing as I never usually go more than 100kmph on the highway would the 12% reduction be a wise move?

I think, if you're always running 35's, then change the CW/P as this changes high and low range gearing. If you only run 35's as an offroad set, then change only the transfer low range gear to a 33% or 49% reduction set as this will improve low range for 4WDing on 35's and leave high range alone for driving on the street.

I will be running the 35s all the time so looks like I need to change th cw&p. 4.75s okay or will the highway revs be too high at 100?

Only knowing that you 90 runs Discovery gearing, it sounds like it would be doing a lot of revs with 4.7's. For a Disco and so presumably a 90 with a V8, standard Disco gearing but on 35's would have you with 4.3's, but seeming as you've probably come from 31's to 35's it would be less of a gear change, withut doing the calculations probably 4.11ish.

Do you want the gearing to stay the same as is when you are running the 35's? The calculations can be done then.

Change the diff ratios and then if you have enough money change the low range gears in the transfer case - it's one of the best modifications I have ever made.