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Old 05-16-2008, 10:20 PM   #1
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34 MPG Grand Prix?

Hi everyone! Great site you all have here. I have gotten a lot of info from reading everyones posts. I finally decided to let everyone in on my adventures in efficiency. I recently started driving my wifes car due to a job change which requires a much further commute. It is a 2002 Pontiac Grand Prix SE with a 3.1L V6 and Auto with about 72,000 mi. Because of the location of my job, I am able to travel mostly on back highways with only about 10 min. of Interstate driving. I keep my cruise set to 55 mph at all times. Anyways, my first tank (filled it slowly to keep foam out) netted me a nice 34.4 mpg getting 458 miles after a refill of 13.3 gallons. Needless to say I was quite impressed! Now, on my second tank, I'm just shy of 400 mi. yet again and still have a 1/4 tank left. I should hit somewhere near the same results as the last quarter goes by fast on this car. I am now looking foreward to trying out some aero mods to see what else can be done. I don't want to go too drastic as I want the car to look pretty much stock. I'll probably tune the motor slightly to see where I can improve on that. I'm looking to maybe get about high 30's with the car. I'm hoping it won't take too much effort.
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Old 05-17-2008, 07:34 AM   #2
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Watch the intake gasket on that 3.1 they are known to fail.
To improve the mileage I would do an Auto-Rx cleaning on the engine. Then change over the transmission fluid to a good synthetic like Mobil 1 or Redline.
When I did an Auto-Rx cleaning on my 97 Volvo 960 I jumped between 1 and 1.5 mpg. The synthetic transmission fluid probably added another 1 mpg.
When it is time to put on new tires get the lowest rolling resistance units you can find.
As far as playing with the engine management GM has got it down for top gas mileage on the highway. I'd look at a minimum to start 25% grill block for another 1/2 mpg or more
If you want to read up on the Auto-Rx head over to bobistheoilguy.com.
bobistheoilguy
Auto-Rx is discussed on the oil additives forum.
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Old 05-17-2008, 07:49 AM   #3
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I would look into some tuning stuff. I know that there are people using either the powertunr or the HPtuners software. There is a tuner guru claiming 45 mpg with a "dangerous tune". Said if he got on it more than 10% it would detonate. I know that he was doing 37 in a SC 3800 without issues.

http://www.3800pro.com/forum/pcms-tu...ng-3800-a.html
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Old 05-17-2008, 10:34 AM   #4
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We will be getting a complete tranny flush soon as it sorta needs one. Otherwise, the rest of the drivetrain is top notch. I will be experimenting with blocking off the grill as I think most of the air to the engine comes from the lower airdam. Also, might consider some belly pans and various places underneath. I won't be messing with the engine management because I don't think it will have much effect on my car. Like stated above, GM has the highway mileage thing down pat on their CPU's.

Intake gasket is fine, I check and maintain the engine myself regularly so no problems there either. I was thinking maybe look into some lower resistance tires when I need new ones. Also, maybe look for an inexpensive set of springs to lower the car another inch or so.
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Old 05-17-2008, 05:25 PM   #5
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I had a 97 Malibu and a 98 Cutlass both with the 3.1L. Both cars my best tank was about 32mpgs and I know I could have gotten better with a full highway trip.

These cars are decent sized and GM can get this kind of mileage out of them. So as far as I am concerned, there is no excuse for the Cobalt only getting an advertised 36mpg highway. It ought to be getting over 40.
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Old 05-18-2008, 04:35 PM   #6
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GM fuel mileage is underrated

The GM V-6 engines--even with an automatic--can pull some pretty decent mileage if you are careful. The gearing in my Grand Am keeps my rev's at 1500 rpm at 60 mph. So, with the highway gearing and keeping speeds down I am consistently getting 34 and 35 mpg. I don't run the a/c unless I have a passenger that needs it and I keep the windows cracked at highway speeds if it is hot. The tires are at 35 psi, which I will probably increase a little as long as the ride is not too much like a buckboard.

I agree with other comments that If we can get this kind of mileage in medium and large GM cars, the smaller cars with 4 bangers should get much higher numbers.
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Old 05-18-2008, 04:44 PM   #7
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I still think that a PCM reprogramming woul help bump you up quite a bit.
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Old 05-18-2008, 05:47 PM   #8
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Quote:
Originally Posted by mjswan View Post
The GM V-6 engines--even with an automatic--can pull some pretty decent mileage if you are careful. The gearing in my Grand Am keeps my rev's at 1500 rpm at 60 mph. So, with the highway gearing
Perhaps that's why I'm so bothered by the low gearing in my VW. I've always owned GMs (though I've driven lots of large Ford trucks for work), and maybe GM just uses taller overdrive gears than everyone else. At 3000 rpm I feel like I'm screaming down the highway pointlessly spinning the engine.

Quote:
The tires are at 35 psi, which I will probably increase a little as long as the ride is not too much like a buckboard.
On my 1997 Grand Am, I had Bridgestone Turanza T 205/55R16 tires at 44 psi. They rode great, handled well, and lasted insanely long. Your '99 is probably heavier and I bet you could get up to 55 psi comfortably if your tires are similarly sized.
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Old 05-19-2008, 07:31 PM   #9
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I think a major factor in the GM V6's getting such great mileage isn't in the engine, but in the transmissions they use. Most cars today use a trans that will hold the RPM's equivilent to the speed they're going, like engine breaking in a 5 speed. But I've noticed on my GP, when coasting from say 55 to 35, the Torque Converter will disengage at around 47-38 mph and the engine will settle at almost below idle speed until I'm on the gas again. It acts similar to other people on here when they put their Automatics into Nuetral. Pay attention to your RPM's in the Grand Am next time and see what happens when you coast from highway speed down to city/suburb speed. The Engine will go to idle on it's own. That's what I would guess is why I get such great mileage. (Until my wife decides to borrow the car and mileage drops down to 28 mpg)
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Old 05-19-2008, 07:39 PM   #10
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I don't have the Grand Am anymore, but my GMC Sierra behaves that way too. You mean automatics from other makes don't coast like that?

Anyway, that's part of it, and another part is tall gears.
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