Tire size evolution

Posted by Ray,IN on February 08, 2003 at 19:25:17 from (66.244.71.4):

The author of this is now unknown, I copied it from one of the YT boards. I hope it is of interest to some.
Tire Sizes and 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 I in tire sizes over the years.

Having worked for Good Year at their tractor tire manufacturing plant in Freeport, Illinois for 17 years, and on tractor tire molds in particular for a number of years, I would like to share what I have learned over the years regarding rear tractor tire sizes.

When rubber tires were first mounted on tractors, nearly all were mounted on eight 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. Now 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. The industry resized the numbers on the tire to reflect the new recommended rim width.

As an example, lets use an 11.25-24 tire as used on the early WC's with eight inch wide wheels. This became a 13-24 when used on a twelve inch wide rim. This seems to be the same exact tire, from basically the same mold as the old 11.25-24, but renumbered to reflect the recommended use on the wider rim, and the corresponding increase in effective tread width gained by i the wide rim. Later on, in the late 50's or early SO"S, the tire manufacturers again renumbered their tires. Now the numbers referred to over all carcass width when mounted on the recommended rim width. Now our 13-24 has grown to a 14.9-24. Keep in mind this is still the same physically sized tire from basically the same mold, just an new numbering system.

A rundown of the evolution of a few typical sizes are as follows:

old 8" rim wider rim new measurement

11.25-24 became 13-24 became 14.9-24

12.75-28 became 14-28 became 16.9-28

13.5-28 became 15-28 became 18.4-28

The tires on each line were the same size, were formed in basically the same molds, but were numbered differently to reflect, first, the increase in recommended rim widths, then to reflect over all carcass width rather than shoulder to shoulder width across the tread. If you will notice, some real old tires will have both sizes on them. I have a pair of Goodrich that show 13-24, replaces 11.25-24. It should also be noted that the tire industry adopted a more aggressive tread pattern about the time the first size change went into effect. Also 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 used exclusively for tractor use came about.

Most restorers are aware of the second change, but few are aware of the first change. They go to the original owners manual under tires and see 13.5-28, and purchase a set of 14.9-28 tires thinking they have the proper original size, not realizing the 193713.5-28, is now actually an 18.4-28. If this seems like 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. You squeeze those beads together by eight inches, the tread shoulders will probably 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 years or so, and if you thought some tractors at shows looked odd because the rear tires seemed small, you were probably right. The restorers used the best information they had, and should not be faulted for this common mistake. Although I am no expert, I am someone very familiar with the rear tire industry , and have come to these conclusions from studying tire sizes on old tires for a number of years.

In conclusion, if a tire looks to be small when put on your tractor and looks odd, you may be justified in using a larger size, without feeling you are not putting an "original" size on it.

Happy restorations

Follow Ups:

• Re: Tire size evolution Hayman 09:45:52 02/09/03 (0)

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