Find a 325i(88-91) that had a 5-spd at your local junkyard. They usually came with a 3.73 lim slip differential. It will make your 325 feel so much quicker all through the powerband. Highway cruising at 80mph will be upped from approx 2450 to 3050, resulting in a top speed loss of about 20mph. Before or after the diff, get a jim C. chip.