Here's an interesting set of 18" tyres: size 9.00-18 with tractor-type treads. Picture courtesy of Dan Eakins, owner of a Canadian
Truck, 3-ton, 6x6, Crash Tender (Ford/Marmon-Herrington MM5-6), who noted "
The tires are interesting as they were made in Canada by Firestone and have an RAF marking on them...."