I think your splitting hairs. There are SSSOOOO many variables it's not funny. Take Interco's site for example, a 35x12.50-15 Trxus MT weighs 66lbs and the 35x12.50-18 weighs 72lbs, but the difference aren't just the diameter of the tire, the 15" tire has 6ply tread and 2 ply sidewall, the 18" version has 10ply tread and 3ply sidewall. Then different manufactures tires have different weights etc. A 15" aluminum rim and tire combo in MOST cases will probably be lighter though. The General Grabber AT in the same size (35x12.50) only has a 1lb difference between them (15-18) but the rim should be lighter. My 15x6 steel wheels and 34" tires weigh over 90lbs a piece together.