RPM's
Do you smell any gas when you nail it and see your gauge moving?
Could someone have drilled in the trunk and gone through the top of the gas tank? I've heard of that before.
Pull your plugs? Are you running rich? Are the plugs wet fouled or black?
My 68 340 Formula S with a 3.91 gear screams at 4000 k at 70 MPH with a 650 Double Pumper. MP .484/284 cam with Rhoades lifters, stock converter and I get better mileage than that. (not sure of the tire size, but taller than stock).
You'd better shut that thing off when you fill up with gas, you'll gain on the gas pump and never reach full.... LOL!
I would check for gas leaks to make sure that it's not leaking out. Second see if you're running too rich. Then chase "more efficient" components...
Could a neighbor be siphoning your gas when you're not looking???