My truck has a 445hp, Cat
(3406E) diesel, I get 6 mpg, traveling at an average speed of 60mph, I come
up with 10 gph. Which gives me 44.5 hp/g/hr.. Is this correct? Bear in mind
I only spent 3 years in 8th grade before I quit.

Scotty

If your foot is flat on the floor, the engine is turning max rpm, with temps
and humidity in the rated range, yes.

More likely, you are using about 1/2 your total hp, assuming you can actually
get max rated hp your engine (not uncommon not to be able to, due to a number
of valid and/or marketing conditions).