I got this E-mail from a good friend of me , so I put this on this Forum for him.
- I have 2 sizes of tires viz. 30 x 3 1/2 [23 inch] and 30 x 3 [24 inch]
- In the spare tire holder of my 17-er is a tire 30 x 3 [ 24 inch]. I wanted to put in a complete wheel 30 x 3 1/2 [23 inch].
- To my big surprise it does not fit. It is quite a bit taller than the installed 30 x 3 [24 inch]
- My question ; where are the 23 and 24 inch in the tires measured ??
- Last but not least : I whish you all a Happy and a Healthy 2014
- Regards Cees Steeman
The rim diameter and width. The outside diameter stays the same front and rear. They could use less material (rubber was expensive) for the fronts.
If the tire is newer the outside diameter could be bigger as some replacement tires are made over over sized. I know the 30 X 3.5 Coker Commander tire I had stood at least an inch taller then the Riverside over sized tires on the car when on the rim and inflated.
The 30 X 3 and 30 X 3 X 3-1/2 inch tires are measured at the outside diameter for the 30 inch measurement. The rim size is calculated by doubling the 3 or 3-1/2 inch measurements and subtracting from the 30 inch for rim size. example: for 30X3 tire (3X2=6) 30-6=24 inch rim size. example: for 30X3-1/2 tire (3-1/2X2=7) 30-7=23 inch rim size. This works for all the early tire sizes on all of the tire brands to find the rim size you need. (there may be exceptions Im not aware of) It does not apply to the 21 inch tires and later tire sizes like model As and others started using in the late 20s.
I forgot to mention the oversizes Mark mentioned. "oversize" was useally added to the outside diameter and width making them fatter looking. If you measure an "oversize" tire it will be larger diameter than 30 inch and also wider. But the size desigination embossed on the tire will still say 30X3 or 30X3-/2, even though they actually are not that measurement. They still had to fit the rim, so the hole size is still the same. Back in the day "oversize" was embossed on the tire. I do not know about todays tires, if they still put "oversize" on them or not.
It sure seem's like the Canadians had a better idea with all 30x31/2 from the word go! I wonder what was/were the actual savings?? Bud.
Today's clincher 30 x 3 1/2" are all 'oversize' meaning they are really dimensional 31" x 3 3/4".
That is just the way it is....you can't buy a new purely 30 x 31/2" period!
When the tire mfg continued to make the clincher tire long after the Model T was gone from the used car sales lots and showrooms, the tire that was provided was the oversize.....that is because it make the most sense and provided the most tire wear and performance for the $.
Note the one at the far left, that was the popular Firestone and what was reproduced in later years and today.
So you have to learn to live with a bigger wider tread.
Found that out when trying to place a tire and rim in my original spare carrier made for the narrow 30 x 3 1/2" std size tire.
New Firestone "X size"
So mounted an original Samson brand in std size and it fit the 'T''