PontiacBonnevilleClub.com

I Have an 05 bonneville gxp, that gets poor fuel mileage. Runs great and smooth. 50,000 miles. Im comparing it to my previous car, that has the same engine and transmission. (2000 cadillac sls). Now i know the gearing is slightly different in the gxp transmission. Is there anything that can be done to improve the mpg? My gxp will get 15 city, 22-23 highway. once in a blue moon it may reach 24. And i dont drive it hard. 65-70 hwy. My 2000 sls would get 17 city, and 26-30 hwy. Ive changed to a k@n filter which did absolutely nothing, and had the flowmaster hushpower mufflers installed.... That gave it an awesome growl, but no change in mpg. So is the lousy mpg due to the lower gearing, or in the computer controls?? ........

Yup, that's about average from what I've seen. My dad has an 05 as well. It's got something to do with the fact that the motor is detuned, and the different body style.

I doubt it has anything to do with the body style or the detuning. It's got to be the short final gear ratio. Short for a V8 car anyways. 3.71:1. Pretty darn good mileage considering.

Im going to take this opportunity to vent some more about GM's stupidity. "Oh no, we cant have anything with more power than the Caddillac" Heaven forbit the sporty marque have as much power as the luxury one. Just like how they protected the Corvette for so many years. Stupid. Apparently the logic is that if a car has as much power as a more upscale one, nobody will buy the more expensive one? Stupid. Cripple one car because another has an "image" to maintain.

Anyway, I bet it sounds spectacular with the Flowmaster on there.

Those numbers are right on. Even a little better than I get on the highway.

John, the cadillacs have the same gearing.

Partially correct. The Cadillac Deville Sedan/DHS, Seville Sedan/SLS, and Eldorado sedan have 3.11:1 gearing. Only the performance trims of the Cadillac models got the 3.71:1, DeVille DTS, Seville STS, Eldorado ETC, etc. Also, the engines are different on the differently geared transaxles. The shorter geared vehicles in the cadillacs usually end up with the higher horsepower engine, while the taller geared transaxled vehicles ended up with the more torquey northstar.

So far, this year, I'm averaging 15.48 mpg with mine.

3,900 pounds, full size sedan, aggressive (short) gearing (which your SLS doesn't have), and any form of "aggressiveness" when driving it, doesn't add up to a "W" in the fuel mileage column.

My commute is 9 miles through backroads here, so I do a lot of stopping/starting, so my mileage isn't as good as if I had been commuting to my prior job in Des Plaines where I was doing it all highway.

Does anyone know if the car can be tuned differently? The car was originally a GM corporate car from a northern state. I purchased it here in California when it had 3000 miles on it. The elevation here is about 500. Now, a few years back I took the car on a trip up to Montana, elevations around 4000 most of the trip. I actually got up to 26. Was this a freak accident? This was on more than one fill up, so It wasnt a tail wind that increased it. So ive always thought that it possibly was set up for a higher elevation? possible or impossible? A trip this summer to Deadwood SD gave me no better than mileage than the 22, Although i was driving faster than normal through the long areas of wyoming. My gps recorded I hit 105 at one point, but who really believes a gps?

It can't just be the gearing guys. I've owned two STS's, one '94 and one '99, both with 3.71 gears, and both could easily average low 20's and cruise at 30 MPG all day long. There must be something different about how the GXP's are than the cadillac's.

I average 20.5-21 mpg in my GXP. Mixed driving. I don't beat it, but I drive it how it's meant to be driven.

alec_b wrote:It can't just be the gearing guys. I've owned two STS's, one '94 and one '99, both with 3.71 gears, and both could easily average low 20's and cruise at 30 MPG all day long. There must be something different about how the GXP's are than the cadillac's.

I think you need to verify what actual final drive you have in the STS. 4T80E's for both cars have the same trans gear ratios.

Final drives however though, they do have differences.
2004 GXP's in GM's service info has two final drives: 3.11:1 & 3.71:1

However a '99 Seville STS has 4 available ratios;
2.84:1
3.11:1
3.48:1
3.71:1

For a car that weighs more than the Bonneville, yet is smaller in size, has equal power (both HP & Torque), my guess is one or possibly several of the following;

- What is your actual final drive ratio?
- Does your Bonneville have additional rolling resistance from bad wheel bearings and/or roll resistant tires?
- While the Bonneville actually weighs less from the factory, do you have the Bonneville loaded down with more in the trunk, backseat, etc, etc.
- Is the Bonneville in top tune? Plugs, filters, etc, etc...
- Being that the Caddy is older, does it have more mileage on it? An older, "looser" motor will spin easier!

I think the poor gas mileage might have something to do with my right foot.
I just have to HEAR that exhaust when I pull away from a stop.

No, really. I'm not just kidding. Driving habits are a big factor in MPG.

The gxp actually uses the same PCM part # as some caddy's of the same year model (can't remember which one offhand) so I don't think it's the tune.

Also, I drove mine to Colorado in the mountains, and got the highest mileage I'd seen with this car by far - 27 mpg. I couldn't tell you why but it was nice.

Here is a chart of the mileage I was seeing on my trip from California to Minnesota.

Andrew, I think that gas pump was messed up when you got 37 MPG. It either gave you
more gas than 3.85 gallons or you coasted down the Rocky Mountains for 146 miles.

IIRC that tank was when he was coming out of the rockies. It doesn't take much fuel when you have momentum on your side.

Ya I was coasting that tank

Those are my numbers and I drive like Grampa Lightfoot. 43340 on the clock.

Well, at 115k miles on the odo I still average 21.5 miles/gallon.