Cascadia makes a 100-watt solar panel for the hood of a Jeep Wrangler. (Looks kind of odd, even for a Wrangler; unless your vehicle is black.) Very handy for off-roading, to power all your electronics ~ even your car battery, in case it gets drained down operating an electric winch to pull you out of a ditch. They also make a wide range of flat, flexible solar panels for cars, boats, RVs ~ any damn where. Those solar panels are flexible enough to go on anything and they’re solar – and built for outside in the elements.
Just think of all time your car spends out in the direct sunlight. If you have an electric vehicle, solar panels on the hood, roof – trunk! – can charge up your battery for some free power. If you think about it, the sides of the vehicle also receive a lot of sunlight when the sun is beyond its zenith. That’s why the Sono Sion electric vehicle will have solar in all the panels of the vehicle!
Even if you don’t have an electric vehicle (Future Tech !), I’m sure they can engineer a system for gas-powered cars that has a much smaller or no alternator (which sucks power off the drive train via the fan belt), using solar panels to charge the battery instead. That translates directly into higher MPG. Even more MPG if, perhaps with an oversized battery (or a second small lithium battery), ALL solar energy impacting the auto panels can be saved and thereafter used to run the electrical systems in the car (instead of, again, siphoning power off the drivetrain).