those mods won't cause a SES light to come on.
if anythign your probably jsut driving a little harder since you now have a muffler and it makes some nice noise while driving.
in city I only get 20-21 MPG, but I have a few more mods than yourself.
highway I still get abt 26 though.

 

I have a stock muffler so I am not driving hard.

 

I've been getting ~33mpg this summer, so I think you have a little room to be concerned. Then again, my commute is 75 miles one way so I don't have a lot of stop&go to deal with.

take it to the stealership or an auto parts store that does free obd2 readings. see if there are any thrown codes you don't know about. wish i had more ideas...

 

I have to second that mileage. Even after 70k miles, my 02 OZ was putting down 26-28 city and 33+ highway. I even had a sustained (350 mile) trip with the air on and averaged about 35mpg at 70.

I doubt you'll see any codes. Put the stock filter back in and BABY the car for a tank or two and calculate your mpg again. If it's consistantly low and you're keeping it under 4k, you might have a problem.

 

has anything else changed, meaning your location (different altitude), different commute (less highway more stop and go)?

 

Just do what PGDlancer said and take it to the dealership, they'll probably find the problem and fix it. I tell man i've never heard of a problem like this on any car before.
But good luck

 

you should just take the car back to the dealership, you must have gotten one of those completely manufacturer defected cars. i dont think it could be considered a lemon since it was new, could it?

 

That actually does remind me of something. Depending on where you live, they oxyginate gasoline during the Summer to keep it from evaporating. That might play a part in that, either because they do and it gets less performance or because they don't and your gasoline evaporates away. I know, this is a long shot far out probably not possibility, but it was a random thought.

And for the lemon law comment, the lemon law was written for new cars, but it states that it has to be a safety issue and that they dealer has 3 tries to fix the problem. Not sure the time period on that, but it pretty much limits it to brakes, airbags, seatbelts, and the like.

 

Well I had the car last summer and it didnt do it. Im still living in the same place so no change in where I live.

The car is automatic if that makes a difference.

 

I'd try changing plugs and oil. Use a good dino oil in your car, as opposed to a synthetic or blend. The oil pump doesn't like synthetic. (At least it didn't in the 02/03 models). Make sure all your plug wires are attached firmly and don't appear damaged.

Since we're kind of grasping at straws here, might as well take all the steps in order.

EDIT: Have you been buying gas at the same station since you bought the car?

 

I just changed the plugs less the 300 miles ago and oil change around 1000 miles. I noticed that when my mileage went down was right when I got my muffler put on and put in diamond shamrock gas. Otherwise Ive always used flying J gas except when I was on my vacation and always stopped at big truck stops to get gas. I use castrol gtx which was in the car from the dealership so I continued to use it.

 

Like I said earlier, I heard quite a few people who complained about their Denso plugs burning out way too quickly, some were even faulty, and even some were gapped incorrectly. Get those checked for misfiring. If you don't want to check that and just want to sit on a computer shooting down possibilities and complaining about it, you're doing just fine.

 

How do I check for misfiring. My bad gas mileage also happened with the stock plugs in as well.

 

Take it to a mechanic. Was it happening because the stock plugs were bad? If so, maybe the new ones aren't gapped correctly. If they were good plugs before and after, then it's not the plugs and therefore I have no clue what's wrong with your car.

 

The stock plugs did not look bad but since I had them taken out I went ahead and replaced them. Maybe as I stated in my topic my car is a POS.