All depends on what engine you have and gearbox you have. Typically if you have a 4.11 gearset you want to go to 4.3. If you already have a 4.3 gearset then you will want either 4.6 or 4.88. If your coming from 3.9 then 4.11 is needed. You need to do the math if your looking for speedo correction otherwise if you after torque you might as well go as high as possible.

 

thanks mate, not sure of any of that info =\..
i've got a 94 gq shorty manual with tb42 carby if that helps?

and just looking for it to drive as well as possible and if it fixes the speedo at the same time, great

 

I'd say you have 4.1 gears. In your case if your running 35's most people go for the 4.3 as it corrects he speedo and you revs at 100km will almost be the same as it is now. Some people go for 4.6 or 4.88 to increase torque but find their revs are much higher at 100kmh which as a side effect give back shocking economy. If your shorty is going to be used for off road work I'd go as high as possible 4.6 or 4.88 as it will give a ton of torque and keep the engine revving higher.

 

would the 4.6 fix the speedo the same as the 4.3?

 

No. For speedo correction you need to do the math and find out exactly what difference each ratio will have. Most people I know don't even bother changing the ratio as the tb42 has enough torque to turn the bigger tyres anyway. They just have to remind themselves that the speedo is out when on the hwy.

Don't forget a crown and pinion gearset will set you back around $500 each end plus fitting (if you don't have your own measuring tools). If I were you I would get your lift in and all the other mods done and see how it goes before changing ratios.

 

cheers for you help bud,

got some more research to do!

 

I had the same vehicle on 35 inch claws. Never even thought about changing diff gears as it wasn't needed and this thing pretty much lived off road.

 

Check the plate on the firewall. Should list the ratio. if you suspect a ratio change check the revolutions of the wheel against the tailshaft. If 4.11's, 4'3's will bring your speedo back.

 

I too had this very question and I can tell you that there is no one answer. Mine was 4.1 geared and a td42 auto. I wanted to get it up to standard gearing with a 33in tyre.

Reading has it plugged at 4.3 being perfect but I wasn't convinced. Today I installed a 4.6 rear diff into my car to give it a test. It accellerates better and the speedo is out by a couple Kay's.

I will run these for a couple weeks before I commit to a rebuild of the diffs and fitting my old lockers to new gears.

Not the answer you want but food for thought.

 

Short answer is every cars different, engine power, vehicle weight, preferred driving terrain all have influences on the "right" gearing.

However a rough guide is to decrease your diff ratio (numerically higher) by the same percentage you increase your tyre size. This will maintain your factory final drive ratios.

For example;

Factory tyre size: 31"
New tyre size: 35"

So the rolling diameter of 35's is 13% larger than 31's.

Factory diff ratio: 4.11
New diff ratio: 4.625

4.11 x 1.13 = 4.644. Closest available ratio = 4.625. Factory diff ratio can be found on the compliance plate.

Another quick rough calc is to just drop a diff ratio for every 2" increment in tyre size.