45 MPG might be tough with an auto. But if you've gotten 39 already, maybe not. I'd probably get around 55 MPG at 55-60 MPH. That's with an HF trans. 1800 RPM @ 60 MPH.
Here is some of the aero stuff I've done to my Wagon:
The most recent is the Wake Trimming Flap (WTF):
