What is the difference between LSD Ratios?

Hi.
I would like to know what LSD ratios mean like 3.91 and 4.10
What does it and if it affects the top speed of the car. I have a 2001 BMW 330ci and I was told to change the differential to increase the top speed of my car. So can someone please answer my question. Thanks!

This is not the LSD ratio, its the final drive ratio. LSD stands for limited slip differential. 3.91 or 4.10 is how many times the driveshaft rotates in order for the wheel to make one complete revolution. A 4.10 gear will rotate faster, thus giving better acceleration but sacrifices top end speed compared to a lower gear ratio like a 3.91.

the math is quite simple, find the percentage difference between your current ratio and the potentially new rear end. Once you know that percentage, multiply the speed at any given gear/RPM by that percentage, and that would be your speed with the new rear end.

For example, 3.91/4.10 = 95.4%. If you are doing 3000 RPM's at 80mph with a 3.91 ratio, at the same RPM with the 4.10, your speed would be 80 X 95.4% = 76.3 mph, a difference of 3.7 mph.

another thing is that you sacrifice some gas mileage with the higher gearing because your engine will rev higher to go the same speed.