Tune up after 29000 miles with auto lite plugs

Is the check engine light on? Tire pressures correct. You have over 200K miles, things are getting worn out.

I don't know what type of plugs you used. If you use the old style plain Jane plugs I would replace them. Platinums and Iridiums are good for a lot more miles than that.

Originally there were iridium spark plugs that were designed to go 100,000 miles before needing to be replaced. Regular copper core spark plugs are good for about 30,000 miles, and platinum spark plugs are good for about 50 to 60 thousand miles. But I don't think that's your problem. Look at your tachometer. Where is it when you're cruising at expressway speeds of about 50 to 60 MPH? It should be around 1,500 RPMs. But I bet it's at about 3,200 RPMs and that's where your gas mileage is going! The transmission in your Grand Prix has a shaft inside that wasn't properly hardened at the factory. The splines break off and the transmission will no longer shift into overdrive and the torque converter clutch will not lock in. You can probably hear a faint whine when cruising too. The service engine soon light will not come on because the computer doesn't realize that the upshift to overdrive has not happened. This is about a $1,500 repair. Hope that helps! Jim

I found my old receipt and they were cheap plugs at $2.78 each. But at the time i was great full for his help.

Thank you for your help G A Arne that makes sense.