i purchased a 2008 pontiac grand prix two weeks ago. it has 16,000. miles

hmm, did u check the fuel injectors?? could be the quality of the gas that bogs down the car. i'd buy some slick 50 engine treatment for gas. i'd try it first then maybe take it to a certified mechanic.

Yes, there is deffinately something wrong with it. Check your MAF, and give the car a thurough tune-up. Is your car displaying a CEL? If so, it could be a bad 02 sensor. They have a strong effect on your fuel trim.

How often do you change the oil? It should be changed every 3,000 miles. Perhaps you should check the air filter too. Also a main factor of gas mileage is tire pressure. I drive a 98 Escort ZX2 stickshift and keep the tires at 35 in the front, 33 in the back. Also traffic effects mileage. The more you have to stop and start from nothing the worse mileage you get. Keeping the car in overdrive is important for mileage aswell perhaps if you do the speed limit you'll find that the car shifts again and revs lower. I also roll down hills in neutral to save on gas. If you have an automatic be sure if you switch into neutral not to over shoot into reverse or your transmission will grenade. Other than that I'm not sure.

ya if it has the v6 then you should be getting atleast 25 mpg in the city if you cant get even twenty then take it to the dealer there has to be something wrong in the computer or something dealing with fuel and exhaust

16k I'd doubt the injectors are fouled that bad... You'd have to get some bad gas repeatedly in that amount of time. Most common cause of low gas mileage in a newer vehicle that I've seen has always been something wrong with the igntion. Plugs with cracked or horribly out of spec gaps. Valve cover gasket leaking into the plug wells... Doesn't always cause misfires but can badly effect spark propagation. I'd start by checking your plugs... If the valve cover is leaking into some of the plug wells you'll know when you pull the wire and the plug boot is coated in oil about an inch or more up it.

Hello, What are you using to average your fuel mileage? If you are using the EVIC, or on board computer, make sure you rest it when you refuel. Otherwise, figure your mileage manuall. Start your trip odometer at zero on your next fill. When you go to fill the tank, take the mileage drive divided by the gallons of gas you put in to fill it. That will give you the average MPG on that tank. The car computers are not always right. If you manually figure your gas mileage to still be that low. Take it to a shop and have the computer scanned for stored trouble codes. Dave

you have a base v6 model and getting 14mpg? there is something really wrong with that, I have an 06 grandprix gt same engine but with the supercharger, and I get 19mpg at my worst when commuting on the DC beltway, usually i get more like 24mpg in regular use and I like to drive fast I'd take it back to the dealer and make them fix it, you should have a good warranty on an '08 I believe the base v6 was rated 20/30 mpg city/hwy and you should be getting it

I have a 2006 with the Supercharged V6 and get around 22mpg. Non-supercharged engines do better and the GTP with the V8 will get worse but I don't think as bad as you're saying. I'm guessing that a 2008 with 16k on it was a former rental car. I'd take it back and question the "certified" part of your purchase. There's no way it should be getting that bad of mileage. Good luck!

David has a very good point, reset the "Average Mileage" by holding the middle arrow button on your dash. (I'm assuming that it has the same computer controls as in 06.

Check to see if the exhaust pipes underneath the car has been crushed by running over something. If the exhaust is restricted badly, this could cause a severe loss in power and very poor fuel mileage