Tractor Tire Size: Find the Right Tire for Your Antique Tractor
A former tiremaker explains the evolution of rear tire sizes for antique tractors
After going to some shows, I have noticed what I perceive as a problem with restorers and the rear tire sizes they select for their restorations.
The problem seems to stem from the progression of numbers used in tire sizes over the years.
Having worked 17 years for Goodyear at that company’s tractor tire manufacturing plant in Freeport, Ill., and on tractor tire molds, in particular, for a number of years, I would like to share what I have learned regarding rear tractor tire sizes.
Early tire sizes reflected rim width
When rubber tires were first mounted on tractors, nearly all were mounted on 8-inch-wide rims, and the numerical sizes on the tires reflected this.
After rubber tires on tractors became more accepted, it was discovered that wider rims contributed to better tire performance. The wider rims actually made the tire wider because the beads were no longer pulling the tire together. Tires labeled “11.25” were 13 inches across at the shoulders. The tread was allowed to flatten out some and the tires became more efficient.
As recommended rim width increased, tire numbers followed – but tires remained the same
The industry resized tire numbers to reflect the new recommended rim width. As an example, let’s use the 11.25-24 tire, as used on the early WCs with 8-inch-wide wheels. This became the 13-24 when used on a 12-inch-wide rim.
It seems to be the same exact tire from basically the same mold as the old 11.25-24, only renumbered to reflect the recommended use on the wider rim and the corresponding increase in effective tread width gained by the wider rim.
Tires renumbered once again to reflect overall carcass width
Later on, in the late ’50s or early ’60s, the tire manufacturers again renumbered their tires. These new numbers referred to overall carcass width when mounted on the recommended rim width. With this change, our 13-24 grew to a 14.9-24. Keep in mind this is still the same size tire, physically, from basically the same mold. It’s just labeled according to the new numbering system.
The tires on each line were the same size and formed in basically the same molds but they were numbered differently to reflect: first, the increase in recommended rim widths; and second, to reflect over all carcass width rather than shoulder-to-shoulder width across the tread.
If you will notice, some really old tires are labeled with both sizes. I have a pair of Goodrich that shows 13-24, replaces 11.25-24.
|Evolution of a Few Typical Tire Sizes|
|Old 8-Inch Rim|
| 11.25-24|| 13-24|| 14.9-24|
| 12.75-28|| 14-28|| 16.9-28|
| 13.5-28|| 15-28|| 18.4-28|