Unladen HGV acceleration?

[lufbramatt]

Random thought this morning as I watched an artic tractor unit with no trailer get away quite sharply from a set of traffic lights. I know some tractor units can be specced with 500-600+ bhp engines, and they weigh about 6.5 tons, so the power to weight is getting close to a normal car. But they have loads of gears to get through and a low RPM limit. What sort of acceleration can they achieve completely unladen? i.e. 0-50 mph?