Simple question. If the Model T was equipped with 30 x 3 1/2 tires, what are 30 x 3 1/2 over-size? I was looking at some old stock and noticed that some of the tires for 30x3.5 contained the wording oversize.
The 30 x 3 1/2 Oversize is actually 31 x 4.
This theory has been posted before on the forum:
Since the 30x3-1/2 was the most sold dimension during WW1, US government thought it was a good idea to ration or ban that size to reduce rubber consumption. Since 31 x 4 fits the same rims as a 30x3-1/2, rubber dealers just sold the larger dimension instead. After the war T owners were used to the larger tires, so tire producers kept on selling them as 30x3-1/2 oversize..
It's too bad that true 30 inch 30 x 3.5 tires aren't being made today. I don't like the way oversize (larger diameter) 30 x 3.5 tires look, especially on front demountable wheels - too big and fat.
All the tires below are 30 x 3.5 - the first two are oversize which is what is available today regardless of brand and the second two are true, 30 inch tires. Notice the difference.
From left to right:
- 1930s or earlier Wards Riverside "blem" or "second" - slight defect in the bead so the Wards Riverside name was ground off - oversize
- 1950 Wards Riverside -oversize purchased new by my father
- 1920s Trailblazer Cord - true 30 inch
- 1920s/1930s Goodyear - true 30 inch
Erik; the Firestone 30 x 3 1/2 tires I just mounted are noticeably smaller than the Wards Riverside 30 x 3 1/2's I just sold with my Buffalo wheels. I don't know about other brands but these seem right.
The Universal catalog lists a 30x3-1/2 ribbed tire, available in black or all-white, with an overall diameter of 30-1/4". Isn't that close enough for government work?
Think the WWII myth is that. The industry was making larger tires, and that provided more benefit and value to have a larger footprint on the T.
The first to do this was Goodrich, and marketing took hold, and others followed.
Here is Firestone "extra size" noted on the left, to compete.
So from the later T years, the Extra Size was the value and it did fit up on the Ford, and also on other cars that used larger 31 x 4 tires too.
30 is the overall diameter 3.5 is the wall height. 30-(3.5+3.5)=23" This is the rim diameter. The oversize tires still have the inner diameter of 23" but have the characteristics of a 31x4 tire. 31-(4+4)=23" This is the more complicated version of what Bill Harper said.
I got some ribbed 30 X 3 1/2" tires from Universal in May of this year. They look to be dandy tires.
The oversize gives a slight overdrive.
Any overdrive effect of a Universal tire with 30-1/4" diameter is so small as to be within the rounding rules. 30.3 inches diameter would be 1%. Where an original tire would give 40 mph, the Universal would give almost 40.4 mph at the same rpm. I get that much variation just from vibration!
Looks like a dog loved that tire
Soapy water used to mount the tire with no tire tools.