if it was long enough ago you where probably loading bias ply tires, they used some sort of fiber belting to hold them together. Todays modern radials use steel belts, and I am sure that makes them heavier than a bias ply.
granted I don’t see many bias ply tires because they were phased out in the 70s for the most part, but the ones I have delt with did feel lighter if memory serves me right
Yes, this would have been in 1972-73.............