identifying rear-end gear

FWIW, if you have an 8.25 with 2.45 gears you will have to purchase a new differnential in order to go to 3.xx gears. In order to make room for the pinion for a 2.45 gear set the diferential is machined differently and won't work with the thicker ring gears in a 3.x gear set.

Also, another quick way to tell which rear end you have is; The 7.25 will have 2 1/2" axle tubes and the 8.25 will have 3".

FWIW, the stock rear end behind a 318 is the 7.25", the 8.25 would have been an option.