Fuel mileage drops like A rock above 1500 RPms, smarty jr

That is gererally how it goes but not that drastic.

Yeah I understand it will most likely get worse mileage at higher rpm but it don't understand a 32 percent drop. Basically running empty at 55mph is the only way to get decent mileage. Anything faster or even pulling an empty skid loader trailer drops it back down to 13ish.

whats your rpms at 55mph? 70mph?
i would assume a 3500 chassis cab comes with some tall gears from the factory

At 55mph I'm right at 1500prm I don't recall where I'm at at 70 but I think its around 2000. My truck has 4:10 gears

Well, I think I figured it out. I cleaned the MAP sensor based of threads I read here. Since mine is a cab and chassis mine is located on the intake manifold and it was super dirty. I'm recently deleted, but the truck had over 100k miles of sucking soot. I've driven the truck enough to know it made a significant difference. It makes sense to me that a dirty sensor would have a higher error rate at higher RPM, thus the significant change in mileage at higher rpm