Well, it depends on multiple factors. The ones you need to consider are the weight on the tires, the size and weight of the tires themselves, and the length of the arm attached to the servo. For now I will just assume that the amount of force required to turn the wheels is known (by measuring it manually). This will be in pounds or ounces (whatever you prefer, preferably ounces since torques of servos are measured in units of that). I will call this value x. The next factor you need to consider is the length of the segment from the axis of rotation of the servo to the rod (or whatever you use) that is connected to the arm. This distance will probably be an inch or two. I will call this distance y. The closer this rod is to the axis of rotation, the more torque the servo will apply to it, but it will travel a short distance. If you have it farther out, it will travel farther, but with less torque. To find out if the servo is powerful enough, use this equation:

x is the force required to turn the wheels (in ounces)
y is the distance from the axis of rotation to the connecting rod (or whatever you use) (in inches)
z is the torque rating of the servo (in inch-ounces)

The "formula": z/y > x

If this is true, then your should be fine. I hope I didn't make it too hard or complicated. I am new to RC stuff, but I hope that my knowledge of physics here has helped. Note also: if you have any other pivot points in your steering setup, you will have to calculate the torque according to that as well. Hope this helps.