318 mpg

I have a 318 with a 500cfm eddie 4-barrel with performer intake. Im lucky to get 13mpg in the city. Block is .30 over with a 340 cam among other things. But runs great. With the stock 2-barrel I was getting close to 17mpg in the city. However I have been told that the reason I am getting low mpg's is becuase of my weak rear-end. It's still the stock 8 1/4 with 2.76 gears. It's almost like they are dragging the engine down a bit since it needs to be upgraded to be up to the engine/tranmission's rebuild. 3.23 sure grip will come this summer. Headers would probably help me a little as well as far as gas mialage.