If your car is an eta engine... using a 3.73 gear on the streets is not advisable, not only is that you will feel as if your running out of gears but your gas mileage will suffer. The right rear diff is a 3.25 or 3.46 from a 535is e-28 body. This will give you the right takeoff...take my word...and ask eta owners around... If you are runnning on the track or racing ...yes take the 3.73.