i want to know how to work out what a diff ratio is.

what diff do u have???

h165 but n general what do you have to do count the teeth or something to work it out.

count teeth on crown wheel and divide it by the number of teeth on the pinion

or just look around the crown wheel till you see some numbers stamped into it....will most likely be the ratio

If you know what vehicle that the diferential came from, & you know it was the original, then owners manuals & workshop manuals can give you the info, particulaly if it's still in the car.

the teeth or diameter of crown and pinion. preferebly the teeth budy

if it is still in the car you can jack up the car ( use jack stands ofcourse) place car in nuetral and mark and turn driveshaft count the number of times it takes to turn wheel one time this is your ratio

Quote:

Do the math! I have a 4.11:1 R&P in a car that has "39" stamped on the ring gear but definitely has 41 teeth.

As for turning the driveshaft, make sure both wheels turn or the spider gears in the differential could throw the count off. Well, now that I've said that I'm beginning to doubt myself. Am I full of crap on that last warning?

No your not full of crap,unless the diff is locked you cant simply jack it up and count revolutions as the spider gears will give a false rotation count.This was all covered recently in another post.
If you dont want to pull the diff out do this:
With the car on the ground,place a chalk mark on the tailshaft and another on the back tyre at 12 o'clock,lie on the ground where you can see the tailshaft and have a mate slowly push the car forward one revolution of the rear wheel till the chalkmark is back to 12 o'clock.Whilst being moved one revolution count the revolutions of the tailshaft eg.3.9 turns..4.1 turns etc.
Cheers

agro4 - I asked the same question a while back, heres the thread....

http://datsun1200.com/modules/newbb/v ... topic_id=32422&forum=1&20