I just did a rough calculation, an average car tire would probably provide about 116 lbs. of buoyancy, minus the weight of the tire and the wheel. Assuming 20 lbs for the tire, and about 15 for an alloy wheel, that comes out to about 81 lbs, or approximately 320 lbs total for a car. This only accounts for the volume of air inside the tire, it might be more or less depending on the buoyancy of the rubber.