gear difference

I have found a guy local to me who has a 8 3/4 gear pot with 3:90 open gears for $200. Price seems right but what kind of rpm would I be getting with the 24" tires, {4000rmp at 60mph} with the 4:56 gears now, how much of a change would it be and would it be worth while to keep looking for a 3:55.
Author: admin