Alright i just rebuilt my engine 351m stayed with same intake and carb two barrel. I put full headers and 2.5 pipe with two 40 series flowmasters. I still get 10mpg. Is it the tire size i have 33x12.5 15s. What is the deal I thougt my engine was tired after 190000 miles so i rebuilt it and thought it would atleast get 12mpg. Whats going on?