I had no idea that you could get both 3.73 (41Ring, 11Pinion) and 3.75 (45Ring, 12Pinion) for the same carrier sizes. I thought that they picked one or the other for a given rear differential. Guess I learned something today, and I agree 100% with Charlie, you'll never notice any difference between the two. The bigger pinion gear may be a little stronger as it spreads the load over a bigger circle.