i never ran a rear finish mower, but have run just about every other type. my answer would be as fast as you want/can without bouncing the you, the tractor, or the mower all over rough ground and still get the quality of cut you want. if the ground is rough, you risk more wear/tear on equipment and you and quality of cut will be poorer if you go too fast. if the ground is smooth, but the grass is high/thick, your quality of cut will suffer. it is more of a feel thing you develope then a exact m.p.h. someone can give you. one of the hardest things on equipment/rfm with going too fast on rough ground is the castor wheel bearings. at the golf course i use to work at, all of the rough mowers we had would go through those castor wheels a bit. if you got going too fast over rough ground and looked back, you could see those castors just bouncing and jarring all over the place on the bumps. that's hard on them. next thing to go would be stress cracks usually near weld/supports. with all this in mind you should develope your own feel for proper speed on the ground you are mowing. good luck.