2000 dodge 1500 318. this engine is not known for gas mileage either. i have 33" toyo tires 16x8 mickey thompsons classic 2 wheels.. i have straight dual rear exhaust with just a cat. drop in k@n. i get 10mpg. when i had factory tires 225 75r16 i was getting 13mpg. hardly any difference. i have heard that if your not pushing a lot of horsepower then headers are not needed. is this true. i also been reading about flow master with dodges. quite a few people say that flowmaster decreased hp and torque. but when they went with magnaflow hp and torque was restored. i like flowmaster for the money, but if that is the case with dodge trucks, poop on it. how about true duals. would that bee better than a y pipe?

 

True duals are good for higher RPM power gains and usually cost power down low. Most shops are using X-pipe designs for the best of both worlds.
The larger tires are the primary power and mileage culprits, more work for the engine to turn over. Headers are usually always good for some power and even mileage although a 10% increase over 10mpg is small.
Assuming you reprogrammed the speedo for the larger tires, I would look into some rear end gears to regain some of the bottom torque you lost. You might not see a change in highway mileage but likely an improvement in city driving. There are calculators that will tell you the best ratio to aim for. Good luck!

 

I have a 3/4 ton with headers, intake, flowmaster, TB spacer.......and I get about 10 miles to a gallon.......even with that I didn't notice much of a increase in mpg.......but I did notice an increase in performance.

 

I plan on going with 3" true duals with an X-pipe and dual 3" glass-packs but I have a Procharger so I guess that's a little different.