ok well im picking up a 318 on tuesday, and the guy is giving me an extra diff, he does not know any info on the diff he is giving me nor the one that is in the car, he just said cruising at like 70-80ish will bring me to like 5-6000rpm's, he was not sure if those were exact numbers but. now he said the extra diff is taller? i guess you would say, he said it will bring your top speed up but acceleration down. so im assuming that its a lower gear one, as in maybe like a 2.93, this will be my first bmw, i have owned well over 30 neons, built lal by me, wlel most of them and like 6 240sx's
now the diff thats in the car is a welded diff.

i guess my question is, how do i figure out the gear ratio's on these diffs, i do not need one thats going to cruise at no where near 5000rpm's as i will be making a 1300 miles trip to WI within in the next 4 months. I would like to swap the new diff in if it is infact a short/long?(im not sure which one it is all he told me is it will give more top speed) because then it should keep the rpm's down and save some gas. also i really do not want to drive 1300 miles straight thru in 18 hours on a welded diff.
it is a 5spd m3 trans so he claims, also how can i tell if it is a actuall m3 trans, the motor is from a 525i if that helps any.