'84 K10 getting 8.9mpg....is this normal?

[KMK454]

33k miles, 305 V8, TH350 three speed, 2WD 3.07 rear gears, stock manifolds, no cats, turbo mufflers. Truck runs nicely, has plenty of power, and starts up with ease. About the only thing it has issues with is every so often on shutdown it might diesel, but it's very rare.

Calculated off of the ODO and gas usage: 10-12 MPG, with or without AC.

Is high? Low? Right? I really don't know... the ODO could be slightly off for calculation, as could the fuel measurement since I can't top-off the tank unless I want to spill down the paint.