what does the ratio stand for in a differential? ex 3-91 or 293 exc?

like 3:11 ratio Three turns of The wheel to 11 turns of the driveshaft

thats how many turns it take the pinion to turn to make one complete turn on the ring gear.for example 3.91-1 3.91 turns on the pinion to one on the ring gear

l FEEL that u had a wrong ratio diff in yr car.and that could be the reason u got them damaged.pls check it out and replace the
recommended diff.

take care

M3MANNY, l TRIED TO CONTACT U OR ADD U,THERE SEEMS A PROBLEM.

Number of teeth on the ring gear(output to wheels) divided by number of teeth on the pinion gear(input from drive shaft). The higher the number 3.91, 4.10 the better acceleration due to higher torque multiplication, lower top end speed. The lower the number 2.93, 3.07 the higher top end speed, less gas consumption. Automatic transmissions will usually have a lower ratio since the torque converter will help multiply torque.

There is plenty info in various Internet sites with formulas etc to calculate which diff ratio to change to, BMW will usually use a 5% increase on the sport version of any model. 10% is usually acceptable for a street/track car.