I assume you are not really asking about the speedometer. I assume you are curious how modifying your cart to go slower on flats (20 MPH) affects the speed of the cart uphill.
It is not an easy answer and possibly not an easy fix. The first things folks will be curious to know is:
- type of cart, year, make model
- series motor, i.e., does it coast freely down hills
- speed on flats now
- other stuff?