225 drinking lots of fuel?

At what speed/rpm were you cruising? What % was steady-state? What kind of terrain?
Hows the engine mechanically? what cylinder pressure? Stock cam?Automatic? % TC slip? Loc-up? What rear gears? Tire size? Number of occupants? baggage? Oh yeah, and timings.Timing is very important.
Point is; theres too many unknowns to hazzard a guess.

My 70 Swinger 340-4spd was 3330 with me in it back in highschool. I was 140ish. With a slanty power-train, I wonder if she wouldnt be under 3000 pounds. You should be able to get some good numbers with that, if the engine is making good pressure.