Take a buddy, and just turn it to find out. My dad and I have turned at least a dozen. Fix one rear wheel so it can't turn (pry bar and foot of the driveshaft counter, or jam it otherwise). Mark the driveshaft w/ chalk or scratch w/ a nail. rotate the other rear wheel 10 times, other person counts the driveshaft rotations. Then multiply the driveshaft rot's number by 2(and divide by ten I spose) and whammo its your diff ratio.

If 10 turns gives you (x driveshaft rotations) just shy of 17, then its a 3.365 17.5 then a 3.54 18.5 then a 3.7 19.5 then a 3.9 Each difference is at least a whole number (except 3.36->3.54, but still 16 or 17?) so this always works for us. Don't forget to grab the speedo gear out of the tranny (one 10mm bolt), even though you don't need it (w/ a 700R4 for a V8 conversion), if you ever sell it it'll help out a Datsun dude.

Others reccomend removing the rear cover plate and readig the ring gear. The number of teeth on the ring and pinion are stamped on the ring gear - so just divide one number by the other. The units I look at most of the time are still in the cars, so this "counting the turns" method works for me.

Ross Corrigan / Vancouver, Canada IZCC#255 '80 327ZX [14 February 1998]