This seems to suggest a
much lighter weight (34 lbs) for a 7.50x20 tyre, but I suspect that it is a lighter construction than
your type of tyre.
It seems from the loads to be for an SUV rather than a bus.
http://www.petestirestore.com/BKT-AS...x20-p2162.html
Using this as a minimum it seems that 50-60 lbs might not be too far out for your tyre.