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Hi --- I keep getting confused. . .

If a tire is, say, 36x4", does that mean that

the wheel is 36 inches and the outer diameter of the tire is 36 + 8 = 44 inches,

or is the outer diameter of the tire 36 inches and the wheel is 36 - 8 = 28 inches?

It seems that the wheel is 28 inches because a 44 inch wheel sounds absurd.

BUT on my 1926 Packard with 7.00 x 21 tires, I know the wheel is alot bigger than 21 - 14 = 7 inches !!

So on a car from the 1920's, the outer diameter of the tire is 21 plus 14 = 35.

But on an earliere car, the outer diameter is 36 minus 8 = 28 inches.

Why is it measured "minus" for early cars/tires and "plus" for later cars and tires??

--Scott

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For the older tires in the format 36x4, the 36 is the overall diameter of the tire. The 4 is the distance from the rim to the outside of the tire. So the rim size for this tire would be 36 - 4 - 4 or 36 - 8 which is 28. so this tire would fit a 28 inch rim. That is the way I understand it.

I can not comment on the newer tire sizes.

Hope this helps.

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Older format was as you reasoned:

O.D. of tire...anX...tire height(measured in inches and fraction)

This would be a large number first and then a small number.

Tires were pretty round at this time so they measured about as tall as they were wide.

Replaced with later format:

tire size(inches and decimal not fraction)...a dash-...larger number(rim size)

Sometimes during the transition the 2 systems were confused and it got into print!!

Today the tire size ( newer method) has morphed into an aspect ratio between the height and width.

Hope this helps but probaby is clear as mud.

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