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Measuring Up Restoration Rubber

By Ben Davidson 
The trend toward collecting tractors from the 1960s is causing some confusion for collectors looking to replace rear rubber on these tractors. The problem lies in the fact that tractors built before 1940 generally have different rim widths than those built from the 1940s to 1955.
Narrower Rims
Antique tire guru Ed Miller of M.E. Miller Tire Company explains that when rubber tires first appeared on tractors in the early 1930s, the widest rear rim available from manufacturers was 8 inches. Small rim widths were used for smaller tires. “With larger tires, the sidewalls were pulled in causing the tire to be very rounded,” he says.
These first tire sizes (listed in the table under the number I) were the approximate tread width measured to the closest ¼ inch.
Beginning in 1938, manufacturers started to employ wider rims for the purpose of improving tire performance. “With wider rims, the tire beads were not pulled in as far, which allowed the tread to flatten. With the wider rim, the same size tire has a wider tread width,” Miller explains.
This second series of tire sizes (under II in the table) still measured tires to the closest inch without resorting to decimal points. By the way, the width of Class II tires was measured across the width of the tire’s lugs.
Another Measurement
In the mid-1950s, a new system was instituted. This method (under III in the table) measured the overall width of the tire from sidewall to sidewall, to the closest ¹⁄10 of an inch. “These tires basically still had the same overall diameter and width as the second (II) method but were now sold using a new number size,” Miller points out.
Miller goes on to say that if the tire you want to replace has two sets of numbers, such as 5.00-15 or 11.2-24, then the first number in that sequence (5.00) is the approximate width of the tire in inches. The second number (15) is the rim diameter.
However, if that tire has three sets of numbers like 26/12.00-12, then the first number (26) is the approximate height of the tire, the second number (12:00) is its width, and the third number (12) is the tire’s rim size.
You can use Class III tires’ older, narrower rims. The tire beads will be pulled in and make your tire look more rounded and balloon-like, resembling the old tire.
May Not Look Proper
The beads on new tires (Class III) are 1 inch high. Flange height on older rims is often 1.4 inches. And tires made after the 1980s have beads that fit over the lip of the flange. A new tire will stay mounted on a Class I regardless of bead height. But the new tire may not look right. In this case, you can put up with the look, try to find tires with the old-style bead, or switch to new-style rims. The last two solutions are iffy, so be prepared to not have an exact fit. 

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