Both really.
Obviously Dual you can only speed by 4/5 mph, but single is most worrying.
Went past one at 44/45 other day, wide straight road.....
Lorry was in looks like a 7.5 tonner though it's so short, except for the hgv markings.
Is the 10% +2 used all that much?

One thing I hate with the lorries, can't rely on any speedo overead to get you out of trouble