How to with video: Find out your differential gear ratio without pulling apart you diff. Diff Questions answered.
Diff gear ratio upgrade

Well I finally got my diff gears upgraded to genuine 3.9 from my slow stock 2.87 I also got a full rebuild and an LSD to go with it.

I'm very happy with it as It's never been so easy to overtake or to climb a little hill.
However,
Having the alloytec with the electronic throttle it's still fairly sluggish in first under 2000rpm. But once u go into 2nd and third it really kicks and makes me go

Thanks to Craig @ diff doc for his great work I would highly recommend him to anyone who would like a diff rebuild or upgrade.

Now for the calculations: WL caprice v6 190kw engine with 5 speed auto.

On the hwy with the stock 2.87 gears I was doing 1750rpm however after the diff upgrade with the 3.9s @ 100 it sits at 2350rpms which isn't actually that bad but keep in mind that I have a 5 speed auto which helps with the lower rpms.

I derived this formula to calculate the estimated rpms @ 100km/h with the new 3.9 gears.

(1750/2.87) x 3.9 = 2.378

Which was pretty close to my reading now as mentioned above which is 2350rpms
That's (rpms @ 100kmh divided by / stock gear ratio) x new gear ratio = estimated rpms with new gears

This is a great way to help u decide between different ratios if your concerned about the rpms on the hwy.

I have also made a lilttle quick video on how to find out your diff gear ratio with out having to pull you diff apart.

Which can be seen here: http://www.youtube.com/watch?v=2y9iq...e_gdata_player

Any questions and or comment please don't hesitate to ask by posting below.

I hope I have answered many diff questions out there and help others

Cheers
Dave.