Overweight
This is an interesting thead. I could bore you with many instances I have witnessed. The answer is simple - DON'T DO IT. If you are lucky enough to get away with it you are condemning someone else to have an incident or accident when the overstressed tyres blow on a subsequent trip or even worse. Sadly sometimes the only answer is to refuse and leave your job, as I have done myself.
Take care out there, it can be difficult sometimes.
MM