The Stovebolt Site Forums The Parking Lot Big Bolts another tire question

my old truck has 7x20's on the back and 6x32's on the front but they look exactly the same. What gives?

The original measurement system coded tires and wheels based upon the tire's outside diameter. The wheel diameter wasn't considered in that. In the early 1930s the National Wheel and Rim Association must have changed their standards. I have a 1931 NWRA catalog that lists sizing using old and new systems, then a 1933 Budd catalog that lists primarily the new system measuring the actual wheel diameter measured at the rim base.

Interestingly, wheel width measurements also have changed over time. Width was first measured across the rim lips, then in the 1940s was changed to the present method of measurement where the width is the distance across the inner rim base. For instance, you sometimes see in the catalogs a 6" (4.33") wheel. Or a 5" (3.75"). So you need to keep straight what system was in use at the time. Stu

Hy guys, to find the wheel size on the old numbering system, double the smaller number and subtract that number from the larger number, example 6x2=12, 32-12=20", hope that helps.

Stu Somehow we need to get those old catalogs you have digitized and post them somewhere; they are a wealth of information.

I think that I am still confused. I get that my 32" tire fits on a 20" rim but will the width now be 4.33" for the 6" designation on the tire?

I'm not sure that 32 on a 20 is that straight forward. I found an old chart awhile ago that has old to new comparisons. I'll see about scanning it tomorrow to my photobucket account and add a link. On the width issue, I'm gonna have to see if the measurement standard went back to the first steel disk wheels. Keep in mind that the 1930s were a time of radical change in wheel and tire technology. Stu

There's too much there to scan and try to keep straight. Stu

Here's what I found covering the two sizing issues. The first is from the 1927 National Wheel and Rim Association catalog, and the second is from a 1945 vendor catalog. The changes made in 1927 are in no way related to the changes made in 1945. It was just that the industry standards changed in major ways these two times. I'm not aware of any other major changes like these until the whole mess that came about in the 60s/70s with metric sizing, "R" coding of wheels approved for radials, and letter/numerical coding of profiles such as G78, etc. Here's the charts that I found. First the 1927, then the 1945. Stu

http://i286.photobucket.com/albums/ll89/truckdog62563/Wheel%20Profiles/rimtireconvertionchart.jpg
http://i286.photobucket.com/albums/ll89/truckdog62563/Wheel%20Profiles/rimtirewidthchart2.jpg

THanks alot truckdog. From my reading of the first chart, 32x6.00 was a 20x4.5 rim and from the second chart, that makes them a 6.50 width since the 4.50 falls between the recommended and permissible.

I think that makes the tire size 20x6.50 even though the back ones are 7.00x20's. They do look a little bulgier.(new word) What the modern code would be is the next step but thanks for your help.

Sounds reasonable. Universal Tire sells the attached 6.50 x 20. No doubt others have them too. Such as Coker, M.E. Miller, and Lucas Tire. Stu

http://www.universaltire.com/truck-and-military/truck/650-20-denman-blackwall.html

Thanks again. You've been a great help in clearing up my confusion...and that's not always an easy task.