A common misconception is that the diameter of the wheel is measured on the outside diameter of the wheel’s lip. What you’re looking for is the edge of the rim, about where it rounds out just before the lip ( above, left). You can see ( above, right) that if we measured this 15-inch wheel lip-to-lip, the measurement we get is 16 inches. Don’t make this mistake.
Of course, there are two things to remember: outside of a few oddball truck and Ford TRX wheel applications, everything is based on inch increments (round to the nearest inch); and you can always doublecheck the tire size, with the third number in your tire’s size being wheel diameter. (Example: 275/55R17 General Grabber UHPs are spec’d with our 17-inch American Racing AR172s.)
[ This is the incorrect way to measure wheel diameter, showing this 15-inch Cragar as a 16.