If solar cells become efficient enough, then the charge speed is infinite/continuous, and free after the purchase of the vehicle. Gas can't beat that, ever.
I can fit about a square metre of solar cells on top of my car. In ideal conditions, with 100% efficient conversion, that's about 1kW. The Volt's battery capacity is apparently 16kWh, so, in theory, in perfect conditions, the panel could charge the battery enough while I'm at work to give me the power to drive home. I'd still have to plug it in there. It still wouldn't provide enough power for long highway trips, or on overcast days. Or for people who work at night.
In the real world, you'll be lucky to average 10-20% efficiency through the system, between panel losses, imperfect pointing, changes in the solar angle during the year, etc.
I could fit enough on the garage to charge the car every day, but then I'd need a whole storage system to charge from when I get home.