Sounds like a cool concept, although the payoff is pretty long. I've heard that photovoltaic cells can put out more energy if you focus more light on them - linearly - as long as you can dissipate the extra heat. What about focusing a bunch of mirrors on a single panel to increase your efficiency for less $$?
I always thought it would be cool to get panels like these:
Amazon.com: HQRP 30W Flexible Solar Panel Power 30 Watt 12V Mono-crystalline PV Module w/ 4 Stainless Grommets for RV Boat Yacht plus HQRP UV Chain / UV Radiation Health Tester: Patio, Lawn & Garden
At 30 watts/495 sq-in, its 1 watt per 16.5 sq-in. If you could find other flexible solar material with a similar watt density and laminate the top surfaces of your car with it...
I figure on the average mid-size car, you could get maybe 20 x 50 inches of area on the trunk, 40 x 50 on the hood/fenders, and 60x50 on the roof - that's 6000/16.5 = 360 watts of power (max). That's power that's always charging the car, anytime its in the sun. Wherever you park it, or even if you are driving.