I need to confirm a quick way of checking a diff ratio.

Mark the tailshaft and mark the rim/tyre and count the number of revolutions the tailshaft does compared to the rim.

Does that sound good?

Yep.

To be more accurate, turn the tyre a number of times (10 is a nice number) . Its easier than trying to work out whether the tailshaft turned 37 or 39 times rather than 3.7 times or 3.9 times.

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1970 ex Group A Rally P510
1971 P510
1972 180B SSS
1965 SP310 Fairlady
1966 SP311 Fairlady

the most spot on way is to read the type o diff and then count the teeth yeah you need to take the back off but hey new oil is always good!!

well if you are going to take the back off.... you can just read the ratio stamped into the edge of the crownwheel - provided you clean the edge with a rag as you rotate it.

ahh i never noticed the ratio stamped on the crown.... but i havent realy play with many diffs well not much more than a swap n change

Now this is a dumb question................Is there a cover plate on the back I can simply unbolt and see the crownwheel and pinion teeth? Or will I only see the crownwheel?

You can take the rear cover off, but you won't be able to see the pinion gear. The crown wheel will have the ratio stamped on it though.

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1971 Datsun P510 SR20DET 276 rwkW
2005 Mercedes AMG E55 350 kW
1969 Datsun P510 L20B EFI 95 rwkW
1975 Ford XB Fairmont GS Wagon 179rwhp
1980 Escort Van SR20DET unfinished
2002 Suzuki GSX1300R Hayabusa 185hp

If you do like Dave suggests, make sure BOTH rear wheels are off the ground before counting number of turns. By mistake, I once had only one wheel free off the ground and thought a 4.8 open diff was a 4.1, when it was actually a 4.8 .

To be sure you are best to look at the ratio on the crown wheel, by taking off the back cover. It may not say, for example 4.8, but something like 39/8 (which equals 4..