Different ways of measuring tires, there have been several over the years.
This article explains it quite well,
a link to the article;
Tractor Tire Size: Find the Right Tire for Your Antique Tractor
Tractor Tire Size: Find the Right Tire for Your Antique Tractor
By Ted Buisker
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
(pre-1940) | Wider Rim
(1938-1960) | New Measurement
(post-1950s) |
| 11.25-24 | 13-24 | 14.9-24 |
| 12.75-28 | 14-28 | 16.9-28 |
| 13.5-28 | 15-28 | 18.4-28 |
Things to keep in mind about tire sizes for antique tractors
It also should be noted that the tire industry adopted a more aggressive tread pattern about the time the first size change went into effect. And, you must remember that rear tractor tires evolved from existing sizes used on aircraft, so there may have been a slight increase in physical size when molds were developed exclusively for tractors.
Most restorers are aware of the second change, but few are aware of the first change. They go to the original owner’s manual, look under “tires,” see “13.5-28,” and purchase a set of 14.9-28s, thinking they have the proper original size.
They do not realize the 1927 13.5-28 is now an 18.4-28. And if this seems a big jump, you must remember the 18.4-28 has a recommended rim width of 16 inches, double the width of the original rim used for this tire in 1937. If you squeeze those beads together by 8 inches, the tread shoulders probably will come in that inch and a half. Remember, the first two sizes are based on tread width; the last is based on overall carcass width.
I hope I have shed some light on rear tractor tire sizes and their evolution over 65 or so years. If you thought some tractors at shows looked not-quite-right because their rear tires seemed small or odd, you probably were right. The restorers used the best information they had, and should not be faulted for this common mistake. Based on this assessment, however, if a tire looks small or odd, restorers may be justified in using a larger-sized tire without feeling they are not putting an “original” size on a tractor.
