Posted by Susanne from adsl-67-124-230-24.dsl.snfc21.pacbell.net (67.124.230.24) on Sunday, July 20, 2003 at 1:11PM :

In Reply to: Wheels posted by Glen from 64-216-188-44.ded.swbell.net (64.216.188.44) on Saturday, July 19, 2003 at 1:12PM :

First, as Jack said, is 1/2" width differene.

Also, the rim size is different. The 30x3 rims are 24", the 30x3 1/2 rims are 23".

The formula for rim sizes are like this. Take the second number and multiply it by 2. Subtract that number from the first (larger number) and that gives you the rim size. So on a car with 30x3 1/2 tires, you can also run 31x4's (seen this done, they used to refer to these as "oversize" tires, the most famous of those being Wards Riversides), and it works. What you get is 1/2 inch more tread on the road, and another inch of diameter.

They are not interchangable. Try to put a 30x3 tire on a 30x3 1/2 rim , it will not seat and could come off (if you could make it work in the first place). Tire is too big in diameter. Conversely, trying to put a 30x3 1/2 tire on a 30x3 rim is worse - the tire is too small for the rim, you'll ruin the tire trying to get it on, or possibly damage the wheel.

It gets worse. For the first few years, from 1919 until about 1923-24, the "optional" demountable rim wheels you could get from Ford were made by either Kelsey or Hayes. (2 separate companies back then). They're *all* 30x3 1/2, but the rims are NOT interchangable. Ford made their own starting in about 1923, but until then you had to have wheels that matched your rims.

Finally... the T tires (until 1926/27, when they went to "balloon" tires - 4.40/4.50-21's) were clinchers. They also made "straight side" tires in the same sizes (30x3, 30x3 1/2), but will not work on clincher rims. The tire pressure on the clinchers was 20 PSI PER INCH OF WIDTH. So on a 30x3 1/2, you're talking 70 PSI. Any less, and you risk cutting the bead of the tire on the rim, and ruining the tire.

I know that's a watch to your "what time is it" question, but now you know.