I once drove back to SF from Castle Lake with a dead alternator by hanging an 85W portable solar panel over the rear window of my mx-5, connected to the battery

I once drove back to SF from Castle Lake with a dead alternator by hanging an 85W portable solar panel over the rear window of my mx-5, connected to the battery in the trunk. It supplied enough juice to run the car with no accessories most of the way back, until a combination of overcast sky and uncooperative route direction started making the car hesitate at the slightest throttle input. So it'd definitely give a little MPG boost by taking a load off the alternator, in addition to keeping the battery topped up like you said. I was tempted to epoxy a cheap flexible one permanently on the trunk-lid. With the trunk-mounted battery it'd be trivial.