If you want to get really technical with the math, you have to calculate for the actual tire diameter, which changes as the tire wears. A 33" tire is rarely exactly 33", and won't remain at that size throughout it's life. For example, suppose a 33" tire has 15/32 tread when new. When it's worn to 3/32 of tread depth, it will have decreased to a 32 1/4" tire (12/32 worn x 2 sides = 3/4"). The Tire Rack website has all the technical data for every tire it sells including actual width & diameter measurements. It's interesting to look at how many 33" tires aren't really 33" diameter.