Fuel economy at 65 mph vs 60 mph

I run at 70 on the interstate. Right in trucks torque band, doesn't "labor", w/ A/C and grossing close to 22,000 lbs nets 10.5 mpgs. For the longest time, many states had split speed limit for semis and cars. Was touted as a "safety" means, but in reality it was (A): Revenue maker for states and (B) a target for semis as cars would come up on them very FAST and not realize it and bam = accident.

If on 2 lane road, I run at 60. Otherwise do posted speed due to traffic or congestion. MPG's here are negligent.