99 Bonne, sumthin' ain't right

Car in question is myDD, a 99 Bonneville SLE, 3800 series 2 V6. I have been messing with it to try and get the mileage up some, getting 22-23 mpg going to work. Mileage is about where EPA says it should be (using the new rating system), but others get a bit more than me, especially the Buick and Olds variants. I have noticed the tach reads higher rpms than other cars and there are different axle ratios available. Searches show that the car could come with either a 2.86 or 3.05 axle ratio (4T65-E trans). Mine has the F83 option code which is 3.05, which would explain the higher tach readings. The lower ratio supposedly hurts mpgs some.

Here is where things get a bit off. Using gear speed calculators (I have used 4 different ones to verify), the car should be doing 74 mph at 2000 rpms (flat ground, converter locked up), but it is only doing 69 mph at that rpm, 5 mph is quite a bit off. The calculators all show that I have a 3.29 axle ratio, which was available in the 4T65-E, but not listed for a Bonneville. GPS shows the speedometer is right, and my scanners live data shows the tach reads the same as the PCM is seeing. Tire size is 225/60/16 (stock size, 26.6" dia), OD ratio is 0.70.

My parents bought this car new so I know its complete history. Did GM screw up and put in the wrong ratio transmission (entirely possible)? The car does accelerate better than the Buicks/Olds ones from a stop. I did also check the speeds vs rpms in drive as well, still showing a 3.29 ratio. I have also found out that if the trans is swapped to a different axle ratio, the computer will throw a code unless it is reprogrammed with the new ratio. What do you think? Is it a Monday morning/Friday afternoon car?

Everything you mentioned is quite possible.

It will cost you $2000 to change ratios. You can buy around 600 gallons of gas for $2000. If your mileage increases from 23mpg to 27mpg, thats....some number that is probably pretty big, I think.

Changing ratios is out, even as nice as the car is they still aren't worth a lot, not spend $2K on it. Mostly curious on how it got this way. Car was a demonstrator, not a special order car per the paperwork. Heck it is GM we are talking about, anything is possible.

There are other things you can do to improve mileage that are cheaper and probably end up with better performance.

I have tried most of the basics and the mpgs stay the same. I have done (or my dad did before I got it): synthetic oil engine and trans, plugs and wires, tire pressure at 35 psi, no extra stuff in car, O2 sensor, fuel filter and even removed the baffles inside the airbox (common mod for these cars). That last bit did let it breathe better, but no affect on mpg. Mileage is better on the highway (25-26), but I drive 2-lane backroads to work. Really just need a good 4-banger to drive.

You mileage is pretty good for what you're driving. I DD a '95 Buick LeSabre with the 3800 V6. I average 19-20 mpg for an 8 mile commute in stop and go traffic. On the highway I can get 31 mpg, but only if I fuel up and head directly onto the highway and fuel up again at the off ramp. Otherwise, mileage averages about 27-28 on the highway. I'm not sure what axle ratio it has, but it is lower than the Olds LSS supercharged we had (I believe the SC cars have a 2.94 ratio). The Olds mileage was always lower the the NA Buick.

Mileage isn't terrible, but the car I had before this one, which I sold to get this so I could keep the car "in the family", was getting 32 mpg on the same drive. That was a 2003 Sunfire 2.2 ecotech 5-spd. It was a repo so it had some issues, but ran good.

after doing all of these modifications, did you reset the ECU? i knew a guy that had one of the supercharged 3800 grand prix, and he said that if you switch to high octane or race gas, you'd have to reset the ecu and drive it like you stole it a while, otherwise the ecu wouldn't add any timing to take advantage (supposedly it will add timing till it sees knock and pull back, then once it learns it keeps the curve, or something. you may have something similar going on with yours, like a tank or two of bad gas and the ecu made some adjustments and hasn't gone back.

in other news i just recently started getting terrible mileage in my xterra, like 16-17 highway from 19-20, so maybe its not just your car, either.

ECU has been reset a while back after the mods and again after the first new O2 sensor failed. Luckily this car likes 87 octane, higher octane does nothing for it. The mpgs have been consistently the same for almost two years so I don't think it is bad gas.

There maybe something going on with gasoline. Been having issues with the Camaro autoxer this past year when it is hot that I didn't have back in 08 when I first put the engine in my El Camino (this is with 93 octane). Lots of problems with volatility and flooding the engine after shutoff. I want to try some race gas this year to see if the problem goes away or not.

I worked for a Pontiac Buick GMC store from 1996-2001.

Drove a lot of different 3.8 models, NA and SC. This is what I found got the best MPG best to worst:

Park Avenue Park Avenue Ultra Regal LS Regal GS Lesabre Bonneville Impala Lesabre w/ Gran Touring Package Grand Prix Bonneville SSEI Grand Prix Grand Prix GTP

You are right on with the Bonneville.

The Park Ave got the best MPG of the 3.8 group, and was the largest. 24-26 combined, 22-24 city, 30-34 highway. Some Park owners claimed as high as 36 with the cruise at 65. The Grand Prix's used the most gas in the city.

I'd like a SSEI myself, preferably a 97-99.

My son's 2000 Impala 9C1 will pull 30-32 hwy averaging 80-85 mph on a long road trip.

I owned a 94 Bonne for a long time. Its MPG sweet spot was 85mph - better than 65. 65 on the highway was like just "off idle"

Strizzo wrote: in other news i just recently started getting terrible mileage in my xterra, like 16-17 highway from 19-20, so maybe its not just your car, either.

In some areas, they crank up the ethanol for the winter gas formulation. Predictably, mileage goes down the tubes...

look for a code on the transmission- they put a sticker with a barcode and a big 3 letter code on the side cover that told the factory people which transmission it was and where it was supposed to go.. see if it matches the trans code on the SPID sticker that should be on the bottom of the trunk lid ( i think).. in the world of GM's, the "M" code is the transmission. it's possible that since it was originally used as a demo car, they might have put a few "ringer" pieces in it to make it just a little better than a regular Bonneville- a deeper gear ratio will make it a bit quicker.

have you done any aero stuff in your search for mileage? those Bonne's have a lot of useless openings in the front of the car as part of their "bold" styling. a little bit of black plastic behind them plugs them up without actually being visible.. lawn edging and self tapping screws make really good air dams. garage door weather seal works great for sealing gaps.. i found about 8mpg on my old 97 Cavvy with just some cheap aero stuff, and 4mpg on an 87 Celebrity by making an air dam and blocking off most of the grille..

my mom's 99 LeSabre gained a little torque and fuel mileage when i swapped out the lower intake gasket in it last summer- i changed it because it had some coolant seepage around the water neck area and at that stupid coolant elbow, but apparently i also accidentally fixed a vacuum leak, too.

The car was a mid-to-late 99 model, so anything is possible. Trans code on the spare tire board is M15, which doesn't really narrow it down since the ratio varies, F83 code is the 3.05 ratio. I will see if I can find the tag on the trans and see what it says. Need to make sure theSPID sticker on tire board is the right one too I guess.

I have resisted doing much to this car, it is in great shape with the original paint and all, never in an accident. Maybe I will try some tricks later, too many other things on my plate. Upper intake and lower gaskets were changed at 31K by dealer, no obvious signs of any kind of leak that I can tell, but a vacuum leak would be tough to find.