Driving at ~56mph gives around 30% better mpg than at 75mph.
So you should thank the lorry driver for turning 4 minutes of travelling (about 3.7miles) into 1.1 extra free miles of journey - If you are a member of the green party.
Perhaps that's it, those that overtake are really environmentalists that want to stop 80+mph hooligans melting the ice caps. Maybe Not.
I think the act you've described should be banned. Actually, while these lazy ****ers are at it, they should ban old people as well... or at least get them to retake their test every year.