Finding out a rear diff's ratio
 

How do you tell what the ratio is of a rear diff?

The only markings i can see are 81 Kn7 and 209B

Is it off the car? If so just count how many spins of the prop flange it takes for the driveshaft outputs to do one revolution

Take the cover off and it will be written on the largest gear somewhere... Well mine was:mrgreen:

as justin said. if it's off teh car, bit of tippex on the splines & case of the output and teh same on the prop flange and case and start counting.

input shaft will either turn just under 4 times (3.9) just over (4.1) or about 4½ (4.444) for every 1 turn of the output shafts.

if it's still on the car, disconnect the prop, mark the input flange as above, then put a bit of tape on a rear tyre and with both rear wheels off the ground, rotate the input shaft