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
