What's wrong with your stock differential? You do know the stock gearing for a manual 535i is 3.46. Going to a 3.15 is definitely gonna make you slower, even with a stick. Although if all you're doing is highway cruising, you should be able to get better mileage. STill, these cars are heavy, so you mileage ain't gonna improve that much. If you want better mileage, GET A HONDA....Originally Posted by grave77