[99LeCouch]

I've come within a hairsbreadth of 40 mpg before in my LeSabre. 39.5 according to my ScanGauge. Same 3800 Series II engine, and the 4T65 transmission instead of the 4T60. If you want to keep it automatic, look at a transmission from a Bonneville SE with the 2.84 (IIRC) final drive ratio. Some cars came with a 3.06 FDR for better punch off the line.

The 4T65 is much more electronically tunable than the 4T60, which is still vacuum-actuated for many things.

My car has a little bit of aero done, and nothing with the transmission except regular service. I noticed a difference with the aero mods to the tune of 2 mpg highway.

My next transmission mod is a shift kit to get it into gear faster. At the same time it will be flushed with low-viscosity Dexron-VI ATF. Both should make shifts faster, and reduce losses in moving a thicker fluid around.