The problems with wheels is that the bigger you go, the more mass on the ouside of the wheel (ie tire and barrel) and the farther the mass is from the center of rotation, the more energy it takes to get the whole thing going (something to do with intertia and momentum). So it's not that the wheel weighs more as it spins, its that the evergy needed to start and stop the wheels motion is perceived as more weight on the car. 4 or 5 pounds on the circumference on each wheel can potentially feel like you're carrying an extra 100 lbs in the car. You'd need to know your physics to figure out the exact numbers, and even then, those calculations are only good for comparing between two different wheel weights and sizes. It's a relative thing, not an absolute thing.