Are the trucks at Prime governed at 55mph?

You can be one of those God of all trucking guys on these forms but as a former mechanical engineer you are absolutely wrong if 55 in your truck runs your rpms up above 13 hundred you most definetely will get worse fuel economy then the truck going 63 running at or below 11 rpms if in hilly environment and you can coast get to at 70 and roll up the hills your gonna get better fuel economy if you really wanna get good fuel economy coast as much as you can