Mileage not good

Car is an 03 GP GT. On the first tank of gas I got 19.5 mpg. I drive about 60% of miles on the highway. I thought I was getting into the pedal too much showing off. 2nd tank I got 20.5. I have been running the a/c about 75 % of the time. Tires are inflated properly. New K&N air filter installed. Car has plenty of power. Car has steel wheels on it. When I bought the car the back passenger side tire was worn heavily on the inside only. The dealer said he had it aligned but I'm not sure I believe him. I read somewhere on here that an out of alignment car will get worse gas mileage. Do you think my low mileage could be an alignment issue or should I look somewhere else?

Thanks

Bad alignment can cause mileage issues. So take the car and have it checked. Also you may find when the AC is not used as much the mileage may go back up.

But will running the a/c bring the mileage down that much? I've seen other guys on here getting around 25mpg.

I'm getting 23 mph mixed highway/town roads with the air-con on, 25-26 mpg for my '99. My '04 is 26-27 mph same type of driving. It normally gets 29-30 mpg.
If you have done all the maintenance on the car, i.e. tune up, tires, and alignment/suspension check; consider having the catalytic converter checked out. I'm assuming your SES is off.
Bob n. just helped me "turn on the lights" that the reason for my GTP's 18 mpg is due to the cat being bad. :slap:
That's after second guessing the PCM setting a P0420 code!

mark

P.S. Hear any squeaking/scraping, etc? Another thought is a brake piston/pad hanging up on you.

No it doesnt have any codes showing. There is some squeaking when I hit the brakes but I assume thats just a worn pad.

Many items can cause mileage to drop. AC, Alignment, Tire inflation pressure, idling in traffic, sitting at McDonalds drive thru and etc. Check and make sure that the plugs are good as worn plugs can decrease mileage too. Also try cleaning the injectors and changing the brand of gasoline you are using. I have seen drops of 3-4 miles per gallon just due to the brand of gasoline I put in my car.

I get 22-23 when I drive human, 18-19 when I'm leadfooted in normal around town driving in my 99 GT. I get 31-33 on the highway at speeds below 75. I have found that setting the cruise at 73'ish is the sweet spot for highway mileage. I've experimented and if I set the cruise at 77, I drop to 25-26 MPG. Maybe you're a combination of leadfooted/jackrabbit starts and highway speeds exceeding 75mph.