[Frank Lee]

There were T's with both tire sizes, though they weren't as different in size as the numbers indicate. 1909-1924 model T's all had 30x3 1/2 clincher tires on 23'' rims in the rear, some were sold into 1926 with clinchers. Front tires were all 30 x 3 clinchers on 24'' rims in USA up until 1919, when demountable rims became an option, then all tires were 30 x 3 1/2.

In 1925 21 x 4.40 "balloon" tires were introduced as an option. They had a new system for measuring size - the outer diameter was about the same as the old 30'' clinchers, but they were named after the rim diameter instead, since that diameter now was more rigid on the tires with steel wíres in the bead, more like modern tires. Balloon tires gave more mileage and were more comfortable (clinchers needs 55-60 psi air pressure while 21 x 4.40 tires needs 27 psi). In 1926 21'' steel wire wheels came as an more and more popular option. They were standard on closed cars at the end of production in 1927.

-a "T" forum