Without knowing how far you've gone, it's only guessing.
I think the worst I ever got was my Hemi RR 4-speed, 4.10s - with its healthy engine, and not caring about MPG, it'd get 8 MPG mostly.
At a steady RPM & light throttle on the highway, maintaining 3000 RPM, it could tickle 10-11 MPG overall.
I could never keep my leg out of it for long...