The corresponding tubeless tire to the 18" tube type is a 19.5". They can be run interchangeably, even side-by-side with each other. The same thing holds true with 20" tube type and 22.5" tubeless. Quite a few bakeries ran the 19.5's on their delivery trucks, at least the ones in the Nashville Tennessee area.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!