The 4.10 is the stock gearing on a 318is. What he bought is an LSD differential (Limited Slip Differential) which is, if from a convertible or automatic, a 15% lock-up and if from an E30 M3 a 25% lock-up (correct me if I'm wrong) That means you get better traction because both wheels are working together, and it's not a full lock because then you wouldn't be able to turn corners since when you corner, the inside wheel has to turn faster (or is it slower?) then the outside wheel. Do I make sense?