I have replaced the ground drive belt with a new John Deere part and installed different engine and transmission pulley. The drive belt still slips. The belt is 1/2 inch wide. What affect would a 5/8 belt have?
B section belt will require a whole new set of pulleys.
Go down a size or two with your belt.
Pulleys wear on the sides so the belts run deeper so they go slack.
You can just keep going smaller till the belts starts to run on the root of the V groove.
