3g hemi gas mileage

My combo is an 03' 5.7 hemi truck engine, 6.1 hemi injectors, cnc made adapters to clear the timing cover so I could use a car intake, TTI headders with their 2.5inch x-pipe exhaust, 742 sure grip center with 3.91's, and 275-60-15 rear tires (28 inches tall). It shifts through all gears and I can feel the tc lock up. The car weighs about 3600 lbs with me in it and a full tank. This is flippin' sad to say the least.

I called Chris at hotwireauto and he said that I need to move the o2 sensor to the passenger side from the drivers side and that my gears are to low, need to be 3.55's. Sorry, but I'm calling B.S. on both of those. I checked the o2 sensor and it was sooty and I cant figure out why.

Are you dead sure the TC is locking up?
Is the tire diameter the same?
How is the exhaust configured on each vehicle?

If a Ram truck that weighs in 5000+ lbs can get 20 on the hwy, no reason why my car that weighs alot less should not get better than 13.5 period.

So it would seem. But the few, but noticeable differences are there and this could be the issue.