9.3 mph

I was getting 10.5 MPG in my 2003 dodge ram ST, It has 33 in tires and a 4.7 engine.
It was missing a bit so I code checked it and 4 plugs were not firing. I got the new plugs and a 3 ft extension for my 1/2 in ratchet and after an hour I got the old plugs out without taking the threads out with them. I bought it used and the plugs looked like they came with the truck.
Now I'm getting 9.3 MPG. runs good but what can cause the thing to lose mileage?

Are there any new codes?

no I took it over to advanced auto and tried it and nothing showed up. I'm not sure what to do, I have been really busy and haven't had time to get back here or work with it.