My new CK30 has Titan Industrial Tires. Shoppintractors provided you with their site. They will have the specs for their tires such as the diameter and load capacity for a particular tire size. You could use any brand as long as the specs are the same. I know Firestone and Goodyear have the same info on their web site. Usually tire dealers will have the books for the tires they sell with the specs of the particular tire brand. Just match them up and you'll be fine. If you want to go bigger, that will take some calculations for front and rear sizes and you'll need to know the axle ratio. The dealer should be able to get that for you.