What size tire you need when the Rim i 23"? How do you measure for clincher rims? where is the start point and what is the end point? Is there any formulas to figure this out I heard there was but I been told a few different things? Are there two different sizes for the front tires and back tires? Also do you have any scetches of where things went on 1918 couplet or pictures we are restoring it? We heard the frame has the wood on the outside? we are trying to make this right? Do you have any pictures ? thank you for your time. Douglas Wilde
Tires in 1918 were of uniform section meaning they were the same width as height. For the 1918 Model year all United States built Model T's used a wood fellow non demountable wheels with 23" rim in the rear and wood fellow non demountable wheels with 24 inch rim in the front.
The rear 23 inch rim used a tire size of 30 X 3 1/2". The tire size worked as folllows:
Outside diameter of the inflated tire = 30"
Section size of tire = 3 1/2"
The math is 3 1/2 + 3 1/2 + 23 = 30.
The front rims at 24" diameter use a 30 X 3" tire size. Outside diameter of the inflated tire = 30"
Section size of the tire = 3"
The math is 3" + 3" plus 24" = 30".
To add to what Royce has said, Canadian T's had the same size on all 4 corners. 30 x 3 1/2". Your car may easily have been converted to the same size both front and back. If it has demountable rims then I would bet that all 4 are 30 x 3 1/2.