TusconJim has it right.
Side notes: All municipalities in the People's Republic on California now rely on traffic tickets as a significant component of their revenue/budget.
I find that with no toad @65MPH and with a toad @ 55MPH the MPG are the same. One can surmise that trailer data would be quite similar.
18-wheelers don't necessarily keep to the posted speed for several reasons, including, but not limited to: Keeping a tight schedule, relying on certain areas which they believe have low enforcement, and trying to keep to the speeds for which their rigs are designed relative to best MPG.
In my experience MOST truckers are great people and terrific drivers. Courteous and careful. If you should choose to follow one of these guys and listen in on communications you should be playing it safe. Your copilot is essential for this.
Caveat: the truck lanes can be most annoying due to the high traffic detereoration of the pavement.
For my own practice, I run at exactly the speed limit for my rig as configured at the time and in the lane just left of the truck lanes. Sure I get some grief from those who just can't wait for anyone, but they can pass at will, generally, and so far I have not received even one performance award from the gendarmerie.
Good luck, and don't forget to smell the flowers. Happy trails.
Spring makes everything look filthy. --- Katherine Whitehorn