...my Park Avenue Ultra has a rather interesting quirk in fuel consumption.
If I do 65 on a flat, I get around 33-36MPG.
That drops down to around 25MPG between 70 and 80MPH.
Yet when I reach 85 to 90MPH, the fuel mileage rises back to around 29-32MPG.
65MPH is around 1700-1800RPM, 80MPH is around 2700RPM.
Same gear, slightly more throttle.
Any theories?

umm maybe when you do 85-90 you are on open road so you stay the speed which is way you get better fuel milage. Just for the record my bonnievile fall milage falls off the shelf at 90. By that I mean I get only 23mpg

idnt the series 2 SC at peak torque at 2700 rpm?

Should have said,
ALL FUEL MILEAGE CLAIMS TAKEN ON FLAT, LEVEL, UNCONGESTED ROADS.

As for the torque peak, that does sound like a distinct possibility.

is this DIC reported? If so it might just be a quirk. Mine says I get better gas mileage at 80 MPH than I do at 60 MPH.

Yeah, that'* according to the DIC, but it does seem to be fairly accurate. I've been watching it for about three months, and it is a consistent thing.