Dustin you need a minimum of an 8" diameter rim, any width for rear and yes turf tires. Most run 18x9.50 - 8 on 8x9 rims. I started running 18x10.5 - 10's this year on 10x10 rims. Much stiffer and less roll and bounce. I still have a new set of the 16x7.50 Carlisle turf pros sitting on the shelf. I couldn't never get them to hook up. My used set are on the junior mower.
Sounds like this is the guy to listen to.
We were racing as a club long before the was anything called ARMA. We have had our pro-mod rules since 2001. No we don't allow kart seats, hoosier tires or centrifugal clutches either. If you want to race karts there are places to do that. We race lawn mowers. We do have a couple ARMA classes as well (SP and Mod-XR). Guess what? they are the smallest classes we have. You (ARMA and USLMRA) just a few years ago started to run we we always had. We were made fun of, called outlaws until someone from the great white north finally opened your eyes. So we are crazy? Said my piece.
I too had several mowers that wernt USLMRA mowers. Those classes are gone now. They were unsafe and slow.
So we switched. I hated the Supermod class when they came on the scene. But that is the way racing is going so I built one. Are they mowers? No. But with the high horsepower engines and the increased speed they are safer.
Sorry if I offended. That wasn't my intention. But when the original poster said it was a Supermod I was confused to the wheel limitation.