Hi, Green Machine.
Thoughts about cruise control: unless you're driving on
perfectly flat roads, the cruise control will not get as good fuel economy as a careful right foot. That said, you may not be up for 500+ miles of closely monitoring the
ScanGauge and delicately feathering the throttle.
You already know that slowing down will get you the largest return, but if you can't or don't want to, then Ben's right: aerodynamic mods are the way to go for your
Aveo.
I'd start with a partial grille block; consider a passenger side mirror delete; smooth wheel covers, rear wheel skirts. Combined, those might net you a 5 or 6% mpg improvement at the speeds you drive. Next undertray, & then start looking at boattailing the back of the vehicle. Have a look through the
List of aero mods you can do to your vehicle.
And - welcome to the site.