We run 25" belts on our planting tractor and the row units do not touch the track, but we are on 120" centers and centered in the rows. Most cab tractors I have been around do not have room to put 20.8s close enough in to stay between 4 rows. I guess if I were going to spec a tire tractor for planting assuming the cab is as wide as the ones I have been around up to now, I would probably go with 480s inside and 600s outside. Should be able to keep that off the row easily and have pretty good rubber on the ground. We run 650s between 30" rows on the combines, but they are close, and they are centered. | 
Ideal Row-Crop Tire Size - 30"
