Hi Indecided,
What are the shift point RPMs now, using the circuit that I sketched, that had 20K at the top of your 5K pot and 4.7K at the bottom? I need to know what the shift point RPM is with the pot set to its maximum, and also with it set to minimum.
If you change the value of the 4.7K resistor to 10K, the minimum pot setting will increase to 1.8 times what it was with 4,7K. But the maximum pot setting also increases 1.3 times.
Then you can change the 20K resistor to 30K, to bring down the maximum pot setting to near what it was originally, and the minimum pot setting also drops, so that it is 1.5 times what it was originally. Therefore you will end up with 2/3 of the number of RPMs between steps in the pot.