scott12180 Posted February 2, 2010 Share Posted February 2, 2010 Hi --- I keep getting confused. . .If a tire is, say, 36x4", does that mean thatthe 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 Link to comment Share on other sites More sharing options...
Guest EMF-Owner Posted February 2, 2010 Share Posted February 2, 2010 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. Link to comment Share on other sites More sharing options...
Layden B Posted February 3, 2010 Share Posted February 3, 2010 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. Link to comment Share on other sites More sharing options...
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