You can start slow plowing and choose a higher gear, see what the tractor can handle. In general you will be in a slow or medium gear.
Plowing is a big power user, and you should match the plow to the tractor. If you need to be in first gear you probably have too big a plow. The dirt might come out more lumpy if you are going too slow, not roll over right.
If you are in a gear you drive down the driveway normally, you are going too fast and have too small a plow. The dirt will be thrown across the furrow and into the next one if you are too fast. As well as rocks really breaking stuff.....
If a 5speed, 2nd or 3rd gear.
If you have tall weeds and grass, have fun after you now it. That mat of debris likes to plug up a plow. Might have been better to plow it still standing, the tall stuff stays attached and flows through the plow. Once mowed, you need it chopped up fine (not sure what mower you used) and spread evenly, and probably dried out well. (Although too dry and thick again will want to bunch up instead of sticking to the ground...) good big counters will help. Most old plows have worn out little nubs of a coulter, worthless.