As John said, there are several different differential drive shafts and ring gears used in the 88, Super 88 & 880. 9/51 ratio is 5.66:1
10/47 ratio is 4.70:1
11/48 ratio is 4.36:1
12/47 ratiio is 3.91:1
13/48 ratio is 3.69:1
What is the model you are wanting to change IE: 88/Super88 or 880? The last characters in the serial number will tell us what you have. You want slow speed for pulling or high speed for tractor rides? Not all of these will enterchange.