with the 55ax i would run either a 12-4 or a 13-4 prop. I run a 12-4 on my girlfriends plane with the stock kit supplied wheels on a grass field and in the spring time it does a good amount of grass cutting. It's not bad in the summer when the grass is kindof dormant. To use the engine to it's potential i would go with tyhe bigger wheels as suggested just for ground cleance with a 13-4 prop.

Dennis