huge question about rear end gears
 

all right. i have a 87 gt mustang. and its practically stock. i have a posi in the rear but not sure of the gearing in it. so i bought a 355 posi, diff and all, from a buddy. but when i took the 355 diff in to have new bearings pressed on. and i was told that is a 330. now i know ford dosent make a 330. but this diff came out a crown vic police interceptor. he swaped out the gears from an open to a limited slip set up, the one that i purchased. the thing is the ring has 43 teeth and the pinion has 13. unless my math is off, which i hope it is. the answer should be 3.307 . i rounded it to 331. and on the pinion there is 331 stamped into it. so i have no idea on what i have. any help would be greatly appreciated. i would like to know what i got or if ford just did somehting really smart that i have yet to understand.

You have a 331. Whatever is stamped on there is what it is.

The newer 05+ mustangs come with 3.55 and some with 3.31s. The 3.31 will work if its for an 8.8 so no worries, however, you will not find a proper speedometer gear to correct a 3.31 gear set for a fox body. The closest speedo gear combo is going to be for a 3.27. Not that big of a discrepancy but its wrong nonetheless.

If your car is stock and hasnt been modified in the rear end you can tell what gear it has either by the axle code on the vin sticker in doorjamb or by reading the numbers on the diff cover tag