The 3800/4T60E can be both good and bad for FE. I have it in my 92 Trans Sport van. It will roll for a very long time and get good mpg, but under any load it is all over. I have gotten 28 mpg to work and back, and am positive I could get 30 downstate and back. I know what you're saying about the 47 mph barrier. There seems to be quite a large ratio difference between 3rd gear and 4th gear. 4th gear does not have much power for hill climbing (unlike my Celebrity which will climb mountain grades in 4th), and once it drops out at 47 mph, you might as well forget FE. Best thing I would do is try to keep it going faster so it does not drop out of 4th.
For the city, a little bump to get it going, then just let it idle to the next stop sign might be the best way to go. Dropping to N probably won't get any noticable gains, at least it doesn't with mine. There seems to be little to no engine braking with this powertrain. Anohter thing is I have never been able to get it to fuel cut, probably due to the lack of engine braking.
I have always wondered how well my powertrain would do in a vehicle that has much less frontal area and less weight. You might just have it. Also your Buick has a sleeker shape.
Sad to say how local driving kills great highway mileage. I know how it is. Last 2 times I have gotten over 40 mpg going downstate and back, then going to work and back till the tank is done makes me never get a 40 mpg fillup.
Just do the best you can. You're doing a heck of a job so far.
