If I may add some suggestions?
Some other factors to selecting the proper motor would be:
1. How much weight are you planning to drive, using this motor. Basically, how heavy of any object? This will lead you to finding what kind of torque you need from the motor.
2. How fast, or at what speed are you looking to drive this car? This will clue you into how many RPM's you will need going to the drive shaft of the car. This will be based upon the torque as well.
3. Finally, given the torque and RPM's, this should give you a pretty good value as to what voltage and amperage motor you will need. But, this will be based upon any gearing that you intend to install between the motor and the drive shaft.
I wouldn't suggest pushing more volts than a motor can handle. While the motor may work for a short while, you may end up overheating the motor, or causing arcs between the armature(s), which will destroy the motor itself. It'll smell pretty bad too. Most motors, or motor manufacturers will tell you the max voltage and current the motor can handle, and usually, they will tell you what kind of RPM's and torque you can expect at a given voltage and amperage.
it may be easier to find a motor that will work with the voltage and amperage that your solar panel will deliver (given direct sunlight). Then see what kind of torque and RPM's the motor will provide. Then you can either adjust the pinion and spur gears to suit the needs of the vehicle, or go with a different motor. Either way, I suggest developing a gear system which will allow you to place different spur and pinion gears into the system, so that you can adjust the RPM's and torque. Take a look at an RC car system, which uses an electric motor. These systems are as basic as they get, and provide measures for adjustment. I used to race RC cars when I was 14, and racing involves adjustment of these factors to improve performance. Thus, this is why I suggest this in this type of system, which will allow you to cover some of the unknown factors.
if you have no clue what I'm talking about (With all due respect) I suggest looking at a mechanics book to find out how motors (RPM's and torque and gearing ratios) work. By mechanics I mean a book which discusses the physics behind mechanical items, as opposed to a vehicle mechanics guide.
I wish you good luck. It may be a little vague right now. A littl trial and error will help you figure the system out.