poor gas mileage

[05grandprixgt]

i am getting poor gas mileage about 20 mpg on freeway on my 2005 grand prix gt. I dont lead foot. i look at the spark plug and they look brand new there are ac delco and with only 42k miles on the car they cant be that old. any suggestions?

[rynoman03]

Run Injector cleaner through your tank like Seafoam or Lucas. Replace the fuel filter, Check air filter for dirtiness \ replace. Check air pressure in your tires and adjust accordingly. I know it sounds crazy but under or over inflated tires can cause bad mileage.

If none of this works look to see if someone can scan \ tune your car. It will help with mileage and performance.

[cerick08GP]

Well, you looked in one of the right places already. Spark plugs are one of the very few tune-up items left on a GP. Sometimes a plug might look good, but actually isn’t. As Rnyo stated, scanning your car is probably your first step if you’re serious. A scan is needed because bad mpg can literally be caused by a dozen or more items. However, on a car as new as yours I would start looking at the obvious first. Look for loose plug wires, vacuum leaks and such. Anyway back to the plugs, I know my car came with iridium plugs theoretically good for 100k. If your car came with platinum’s, I would change them since they typically last 50k. Of the other items to consider, one is proper tire inflation. GM wants 30psi in your tires, well IMHO that is the lowest pressure you ever want. I run my tires at 32-36, low end in winter and high end in summer. And on the freeway use the slow lane and your cruise control. And use the A/C sparingly; it takes a noticeable bite from anyone’s mpg.

How are you calculating your mpg? Because to be honest most people haven’t a clue what they are really getting. Also fyi, your car’s DIC mpg average read out could be off by a few ticks.

[05grandprixgt]

i was told 05 have fuel filter in tank is that true? tire are fine i forgot to say that the car is sitting on 18 in rims will that make a difference? i am just going off the dic and on what my old car got i had a 95 trans am auto and with 20 bucks i could make it back forth to work all week with this car i have to put 20 bucks in every three days and the gp is supposed to get better mileage

[cerick08GP]

Yes your fuel filter is in your tank, and shouldn’t need attention for at least another 50k. Unless you’re pouring sand in the tank, replacing it shouldn’t be an issue for a few years. And 18 inch rims don’t affect your mpg much, but the width of your tires can slightly affect mpg. Going wider than stock will slightly decrease your mpg. Also if you changed the total OD of the tire your speedometer and DIC will be off, which could explain your readings. Maybe. Your car should get about 23 mpg average, and about 30 mpg on the highway. You need to start tracking your mpg for real, because it doesn’t sound to me like you really know at all what you were or are getting. To be honest, this site gets several new people a week just like you which is why only two people bothered to answer you. The deal is your GP should get 2-4 mpg more than your old car, assuming there is nothing wrong.