Thread: Horrible Gas Mileage

Hello everyone. I'm new here. I just purchased a 2000 GP GT a few weeks back. Engine 3800 SII, 171,000 miles on it. The last two tanks of gas I put in it, have barely been getting 200 miles per tank. I don't drive it hard. Replaced the MAF sensor when I first got the car. It runs great now. Sounds good, shifts good, and drives really nice. However the gas mileage isn't very good. Around 16-17 mpg. What are some things that could be causing it to be this low? I'm no mechanic, but I ain't no girl either, I need some help trying to figure this out.
Has new plugs and wires, AC Delco iridium. All new tires, all aligned properly and inflated properly. New mass airflow sensor, new airfilter, and changed oil. My dad suggested that it may be a problem with the thermostat. I'm pretty sure it has a 185 and not a 195. So I'm going to pick up a new thermostat and temp. sensor tomorrow, hopefully if fixes the problem.

Anyone have any suggestions as to why my MPG isn't good?

What kind of driving do you do? If its all city then that's about right.

I get about 18 mpg in mixed driving on the 2000 gtp using premium of course...

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Well Cody, you have already hit most of the big ones for restoring mpgs. The only sensor you missed is the front O2 sensor, which just happens to be the most important sensor related to fuel economy. At a 171k I’d guess it was replaced long ago, they really only last about 120k. The front O2 sensor isn’t expensive or hard to replace just so you’re aware. BTW notice I said front O2, not the rear O2 sensor.

The thermostat could be your real problem; I’m in agreement with your father. A colder than stock thermostat could cost you mpgs.

Also, be aware that after any sensor change out the car will take a while to re-tune the car. I don’t recall the exact process it goes through, but it takes like 50-ish miles. So don’t judge a change until the next fill up.

And of course driving habits, if you mainly drive short trips in the city you are going to get crappy mpgs. GPs get very good mpgs on the open road, but city driving kills it. And this is really true for most any mid/full sized car. The real mpg test is by setting the cruise at highway speeds at about 55-60 mph, and if you’re not averaging over 30 mpg on a flat road you have a problem with your car. If your getting over 30 mpg, you have a problem with the driver.

BTW welcome to the forums.

The thermostat is supposed to be 195 degrees stock. If it is stuck open, it might hurt your fuel economy, but not by much. Are you carrying a lot of weight in the back/trunk? Might be a factor as well. Also, you might consider cleaning the throttle body and getting a new upstream O2 sensor. But, these are only mechanical factors. Your fuel economy is just about right if you've got a heavy foot. The DIC reports about 20mpg city/34mpg highway in my 1999GTP (performance shift OFF and driving conservatively), however this number is probably off by 2mpg for each.
Also, if you keep pulling the battery cables to "fix" things, that can hurt your FE as well...the car's computer needs to relearn every time you do this.
Just some food for thought.

EDIT: Haha, looks like I was just a second slower. Listen to the guy above.

Originally Posted by CDooley

clean throttle body, check for vacuum leaks, maybe swap the PCV if you havent already. also use copper spark plugs, these engines like those better IMO

Imma teach ya something.

Stock cars love Ac delco Iridiums.

Modded cars love copper cores.

Try to keep that car around 2000rpm and under at cruise. It likes the mpgs at this range. The o2 sensor is a very good idea as well. Also it may not hurt to unplug your battery for a few hours. With your map and maf being replaced you probably need to reset your fuel trims. Without a tuner this is the only other way to do it by killing the power to the pcm for a good few hours.

Originally Posted by Cody Benjamin

Thanks for all the info and help guys. I forgot to mention a few points. I drive this car just like my Grand Am. This car has alot more power, but I still try to accelerate slowly and I don't ride the brakes hard. I don't go any faster than 55-60 mph. I drive 15 miles one way to work, at around 55-60 mph. No city driving, and only 5 stops/turns before I'm at work. I don't have any extra weight in the car.

Assuming all that you have said is accurate, I agree you are getting pretty bad mpgs. You “should” be getting about 22-23 mpg avg not 16-17 mpg.

Once you get all the work done let us know how it turned out. If you are still having issues we can go from there.

And the battery question, the best time for that would be to disconnect it over night. Honestly I wouldn’t sweat it, your car self tunes constantly. However I must put in a disclaimer, I’m no tuning guru. That said, on a stock car there seems to be much less of a need to reset your fuel trims after a sensor change. Your WOT fueling might be a little rich (at least for a while) but that is beyond your mpg concerns. I’m not saying don’t reset, I am saying don’t lose sleep over it.

probably