I constructed a spreadsheet that calculates Tire height vs Differential gear ratio vs Motor RPM vs Cart speed for me.
Without the motor being RPM limited by the controller and in a typical two-passenger PDS cart with a top, but no windshield, a stock, standard speed 36V PDS motor will spin at about 5600RPM with a 36V battery pack, which is 24.1MPH on 18" tires.
If up the pack voltage 36V to to 42V, that is a 16.7% increase in voltage, which ups high end torque by a similar amount, so the max rpm also increases by roughly the same percentage as long as the mechanical drag stays the same. That means the max RPM should be 5600 + 16.7% or 6532RPM, which is 28.1 MPH on 18" tires.
If up the pack voltage from 36V to to 48V, the pack voltage is increased by 33.3% and the max RPM climbs to 7465RPM, which is 32.1 MPH on 18" tires.
When you increase the tire height, the cart travels further for each tire revolution, so the cart speed is faster at a given RPM, but increasing tire height also reduces the amount of torque available where rubber meets road, which is effectively increasing the mechanical drag on the motor, so it will not reach as high of a RPM for the same applied voltage.
Due to the field weakening (Field Mapping) process, a SepEx motor's Torque vs RPM vs Voltage curve isn't linear, but if it were, the max RPM would be rough the same percentage lower as the torque loss caused by the increased tire height.
Based on that assumption, the 10% torque loss of 20" tires would limit the motor to about 5879RPM at 42V, which is 28.1MPH on 20" tires, and about 6809RPM at 48V, which is 32.9MPH on 20" tires. However, the high end torque of a SepEx motor is not linear and the max RPM reached at 42V and 48V will be higher. I just don't know how much without having access to the actual Torque vs RPM of the specific motor.
Since it is a direct drive system, the motor RPM vs Cart Speed is 232.3RPM per MPH for 18" tires and 209.1RPM per MPH for 20" tires, so if you know the cart speed (GPS), you can calculate the motor RPM.