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Some think 4.77 are too much if you drive on the highway a lot. It will raise your "cruising" RPM in any given gear, and if you are already in 6th gear, there is no way to reduce the RPMs. (Except slow down.)
The stock 4.10 Gears are made by mazda (I believe) and are used by MANY manufacures. Similarly designed gears in different ratios are also available. These would be the commonly available 4.44 and 4.77 gears.
The 4.56 and 4.57 gears were developed by Richmond gear. They are stronger than OEM and fall into the range between, not much more than stock, and too tall to cruise on the highway. There tends to be a LITTLE bit of whine associated with these gears, which is permanent.
Any of the gears will whine if not installed properly. It's trickier to setup than (apparantly) other diffs. Find someone with a LOT of experience installed any of these gears, or you are likely to experience failure.
4.56 on the right, 4.77 on the left.
Notice the thicker teeth, and the thicker ring gear on the 4.56