Careful drafting semi's.
It hurts their mileage, plus they get annoyed seeing flickers of a little car behind and not knowing if you are going to hit them or not. Under certain conditions, they CAN out-brake you and they will not even know you hit them.
Some of them get a kick out of it though.
Watch for signs that they may not like it, careful, it may not be subtle.
On the other hand, drafting them for a bit to figure out if they are speeding is great. Do that, then haul ass up to the next line up trucks... I used to do this driving from Phoenix to So.Cal. and could average 85-90 safely and get decent mileage. If they are speeding, go for it, if not, slow down. After a while they will catch on, and radio ahead for you (truckers are bored, they love anything even remotely entertaining), if something happens a truck may block your path. This is done to save you from a cop.
Be good to the trucks and they usually will be good to you.