I got a 09 ram 1500 it has an exhaust system and that's about all for performance and I only get about 12 mpg on the highway, I have changed plugs air filter oil and still the same. oh and I have 35" tires on her. for the life of me I cant get the mpg back up. it has been this way since I bought it and no codes.

 

You already answered your question. 35's.... That sucks up your gas mileage. Im guessing you also didn't re calibrate your speedo. That is also going to throw off the computer's MPG Calculator.

 

What he said. Big tires=big fuel consumption. Ya gotta pay to play.

 

You answered your own question if it's been that way since you bought it. That's as good as it gets with the tire size you are running.