34 MPG Grand Prix?

[adamsjd1]

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.

[ALS]

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.

[Dust]

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

[adamsjd1]

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.

[karnovking]

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.

[mjswan]

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.

[Dust]

I still think that a PCM reprogramming woul help bump you up quite a bit.

[theholycow]

Originally Posted by mjswan

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.

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.

[adamsjd1]

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)

[theholycow]

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.

[adamsjd1]

I know our Caravan doesn't do that. Nor does any other Automatic I've ever driven from another maker.

I've got a coworker who drives a 1996 Buick Park Avenue with the GM 3800 Supercharged, like what's used in the GTP. He consistantly gets 30+ mpg on that car as well, even with the higher fuel injectors and supercharger to suck up gas.

[almightybmw]

mmmm, just registered, but have been reading for a month or so, I have to chime in. I also have a 02 GP SE, and on the last trip out west, one leg of the trip netted me 34.88 mpg. Cour De' lane to Portland, cruise set at 67mph. I'll bet I could break 35mpg with a few mods, as that was all stock, all driver skill.
I agree with the trans comments, as at 67mph it's just a hair under 2000rpms. at 42mph the 4th gear lockup engages and I can pretty much slowly get to 67 from there. The off-throttle disengage is nice too, don't have to shift to N when coasting.

Now I just need help with the in town, as 20-21mpg just isn't good enough for me. I do bike when I can, just need the mpg up when I do drive.

[Dust]

Just thought of something. Take a look under your car, there should be a nice big bump in your exhaust where your 02 sensor is. If you put a straight pipe in there, and mount the sensor sideways, you should pick up a mpg or 2. It's called a u-bend delete.

[bowtieguy]

i had a 1995 olds 88(3800). things that helped me were: grill block overinflated tires(55psi front, 50 rear), and synthetic fluids, besides driving tech of course.

realizing these were mentioned prior, just wanted to confirm them.

however, be wary of the tranny flush. inexperienced shops often perform this with electrical problems to follow. besides, many(most?) mechanics deem it unnecessary.

whatever tranny service you do, by all means DROP THE PAN and REPLACE the FILTER!!!

[almightybmw]

Originally Posted by Dust

Just thought of something. Take a look under your car, there should be a nice big bump in your exhaust where your 02 sensor is. If you put a straight pipe in there, and mount the sensor sideways, you should pick up a mpg or 2. It's called a u-bend delete.

From what I've read the U-bend delete doesn't apply to the 3.1, only the 3.8. I could be wrong, and not getting 1-2mpg more. Guess I'll have to read more and find out.

[adamsjd1]

I believe the U-Bend delete is also only for the 3800 Series II engine.

Today I checked my air pressure with a gauge and saw that all my tires were at 30 psi or less. One of them was at 25 psi. So I've now them all equally at 45 psi. Of course, the ride is a little firmer, but I've noticed the car rolls much easier. I should be getting similar, if not better this tank. I'm halfway beyween 3/4 and 1/2 tank right now with about 260 miles on the trip odometer. Should be interesting when I fill.

I might look into creating my own belly pans made out of Carbon Fiber as it's lightweight. Shouldn't be too difficult to create a mold with a few bends in it to get the pans where I need them. For permenant places, I'll simply pop-rivet them into place, and then use low-profile bolt heads on removeable ones.