Coyote is on the mark by saying: "it depends".
To the replies stating "categorically, NO!": I have driven my car a fair bit on a solar PV charged 12v system. Not with car-mounted panels, but by depleting the battery for local driving, then recharging via PV when parked (including aiming the car at the sun so I could leave prop a panel up in the windshield).
You can get away with smaller PV area, as Coyote says, if you drive infrequently and have lots of time between battery depletion for recharging (my situation).
If you're trying to size your array to generate the necessary amount of power to run the car
without depleting the battery, then it becomes much more challenging.
An 80 watt 20v consumer grade panel is about 8 square feet. You'd need two of those at a minimum just to support the electrical demands of the engine - ECU, fuel pump, injectors, occasional brake & signal lights (no other electrical accessories on, including lights, wipers, radio etc.). And they'd need to be perfectly aligned to the sun to generate their peak power. Change any of those factors, and you need even
more panels.
Check this thread for more info
http://ecomodder.com/forum/showthread.php?t=98
And
http://metrompg.com/posts/alternator-optional.htm