A CAI will cause more fuel to be dumped to richen the mix as more air is entering the TB, modest power gainer but can negatively effect mpgs by about 1 mpg. The colder the air introduced, the more fuel is dumped resulting in more power, less mpgs. If warmer air from close the engine is introduced, less fuel is dumped, resulting in less power gain, but less of a negative effect on mpgs.

Power Wire is fooling the PCM into thinking the engine temp is actually cooler than it really is causing open loop and again, the PCM will dump more fuel again resulting in a small power gain but a negative effect on fuel economy, usually by a mpg or two.

That could be 2-3 of your lost mpgs right there.

Some power adders also will benefit your fuel economy such as an e-fan and also those that will lean your A/F ratio while adding power such as LT headers and a tuner that advances timing. Some negatively effect fuel economy like the Power Wire or a cooler thermostat while giving power gains.

Dirty Throttle body and a dirty EGR (which won't necessarily throw a code if it's not completely failing) are also very common culprits of mpg losses...

 

I'm just going to throw this thought out there...tire pressure good

 

I noticed that you changed 8 plugs. If you have a Hemi, there's 16 plugs. Change them all and check the gap before you put them in.

Dirty throttle body?
Air filter?
Crappy gas with too much ethanol? (ethanol produces less energy per gallon than gas).
Tire pressure was already mentioned.

Did you change something right when the mileage changed - added CAI, changed tire size, something like that?

That's all I can think of.

 

^^^ he has a 4.7.

clogged cats, bad o2 sensors (front usually), and the power wire could all be factors

 

How do I clean out the cats? and is it hard to change an o2 sensor?

 

i noticed a couple mpg loss when i switched plugs to those Champions also..

 

I way also going to suggest tire pressure. You dont clean cats out though. They have to be replaced. The O2 sensors are about $25 each and you can get them at places like Autozone. Also, strong winds can also cause issues of less mileage. You can try resetting the PCM also to clear its memory of your driving habits.