I have had a number of vehicles with the 3.8 engine and you should be getting that kind of mileage at highway driving below 70mph. It could be transmission related, but there are other possibilities.

Did you use non-alcohol gas this time? My switching to NA could jump the mileage by as much as 10% Have you replaced the air filter or any other parts? Do you have new tires, any brake work lately?

As I said, that is the mileage you should be getting.

Gift horse! Stop looking in its mouth!

Downhill both ways with a strong tailwind.

All new brakes ,five months ago.
83,000 on the odometer

I’m going with the torque converter not locking up.

Will the torque converter not locking up be an intermittent
Ongoing "thing" ?

Spoke with the transmission shop today
They said that improved milage after
Automatic transmission repairs is very common.


I will be testing this again in the near future.

A 25% increase in gas mileage will pay for the transmission repair pretty quickly. Will be interesting to see if you can verify the results.

It may take a bit longer for me,
I only put between 4,500- 6,000 miles on per year .

I am "that guy"
You know , the one that has to purchase new tires
while the old
ones still have a third of good tread still on them

quote:

Originally posted by Greymann:
Sigmonkey answered your question.

And, the Sigmonkey knows of what he speaks about Transmissions!

Have you considered the torque converter condition prior to rebuild?

No, prior to my trans. Rebuild I
did not know torque converters
Had an effect on anything


It was shifting hard from 2nd to third,
This.

FWIW I owned three 3800/440T equipped cars, two Bonnevilles and one Regal. From the best I can remember the torque converter didn’t lock up until around 55 mph, if you were trying to feel it happen it was pretty subdued, a good indicator of that occurring was to watch the Tach drop was reached. Disengagement was through speed drop below 55 or depressing the brake pedal or both.

If you have a tachometer, for the same road speed, is there any difference in rpm? Have you noticed? Transmission slippage or T/C not locking up > higher rpm for given road speed > lowered gas mileage.

Because of these cars' "torquey" engines, "tall" (higher) gearing and rounded, relatively streamlined shape, high 20s highway mpg is common, with 30 achievable. It has a couple of cooling system-related quirks, a tendency to leak oil and the automatic HVAC vent control "programmers" like to go bad, but is generally a pretty much "bulletproof" car.

Rust got to all three of mine.

This car needs a half a quart of oil between
Oil change it's been that way since I got it.

I like pretty much everything about the vehicle
,
Especially the head lights, the horn, the seats
Are very comfortable, great trunk space.

They really did a very good job with this car.
EXCEPT!

There is a huge low hanging beam between the
front wheels that prevents you from going in
to driveways , straight in,

It's mandatory that you put one wheel or
the other up over the driveway lip or
BANG, that beam will catch.

( I know a guy here in tiny town that
has three Lesabere's )

^^^ Yeah, rust is about the only thing that kills them.

I remember one that had one of those cooling system problems, related to air in the system combined with Dex-Cool coolant and neglect. (Take any one of these out of play and it would have been OK.) The coolant turned into a rusty soup (if you can imagine that). Several power flushes and even taking out the radiator so as to force water under pressure through it would not get it clean, and it even ate the impeller off the water pump. The engine still purred and even passed the combustion leak dye test.