340 Gets Terrible Gas Mileage

I had a 460 ford and got 7mpg in the city and 13 on the highway.

6 does seem a bit low, i would expect 11 to 13 in the city.

BUT... There are way too many variables here.

Is your spedometer correct?
Do you have a high numerical rear gear?
Is the engine runnng rich?
Do you have a fuel leak?
Do you have a misfire?
Do you gun it from stoplight to stop light?