some things to think about:
is odometer accurate? changing tire size may change odometer accuracy.
wider, stickier hi performance tires will yield lesser gas milage than narrow, harder compound tires.
if your imrc is not working correctly, that can affect milage.
after that, there are a lot of things engine related that could affect things. clogged cats, bent exhaust pipes, spark plugs & wires, air filter, egr system, etc.
could be something mechanical:
maybe your brakes are binding up or you have bad wheel bearings.
