Randy, it is not an exact science. That being said there is a chart on the Slot.It site and in their catalog that maps their tires to wheels and shows how a particular rim OD (outside diameter) affects a given tire’s installed OD and Width. Naturally you want a very tight fit, BUT not so tight that the tire “crowns” (i.e. a convex buldging center and withdrawn sidewall, or a concave center ~the opposite).
Also – keep in mind that for most of our races many people choose the following: Slot.It F30, (SIPT28), NSR Supergips, or BRM Shore-30s (part#s provided in another post). Some guys use Slot.it F22s, N22s and NSR Ultragrips esp with high torque (>170gm) motors.
’30s’ have a durometer rating of their softness of 30-shore. F22, N22 are 22-shore (softer and hence shred easier). Supergrips are between 25-30, maybe 28. Ultragrips are 18-22. (NSR is not explicit on the Shore rating for their tires.) F15s are typically only used in the winter when the track is below 60deg. They may last a Thurs night and one hard race.
** I am sure that some of the other guys will have some good comments on this topic!
The Slot.It chart looks like this: https://www.bing.com/images/search?q=slot+it+tire+chart&view=detailv2&&id=A5ACB3DD9B90817CF455F70AD0D055ED793F424A&selectedIndex=0&ccid=FPuJpx1Z&simid=607992530833834800&thid=OIP.M14fb89a71d59474544caba74acaac25eo0&ajaxhist=0
"... get on your bad motor scooter and ride!"
Sammy with Montrose