Does my /6 sound right?

Well I recently went on a trip that was a round trip of like 231 miles.
I calculated the MPG and I got 12 MPG and this whole trip was almost all highway.
Whats weird is the speed limit is 70MPH and the fastest I could go was 66-68MPH. That was flooring it the whole way!
I am assuming something could probably be wrong if I am wasting that much gas and I had to floor it the whole way.
Does this sound about right?

Specs on the car:
The car is a 225 /6 (single barrel carb), factory exhaust, it has a 7 1/4 rear end, auto transmission, and points ignition.
I did clean out the whole fuel tank a few days before and filled it up with new gas.
I changed the oil in the car a few days before(10w30 castrol and purolator One filter), replaced fuel filter (purolator), replaced thermostat/flushed cooling system, did alignment, rebuilt brakes/properly adjusted them, and I inflated all my tired to 32 PSI.

I am just a bit confused at the moment on what could have been wrong to waste that much fuel.

Any ideas?

Thanks!