Dom-
A solution to this problem could be easy, or could be difficult. We can not be very helpful with out more information from you. I would like to know more about the servos you plan on using to power the wheels. Do you have a website that provides information about them?
There may be a simple misunderstanding. A servo to me is a small device, containing an electric motor, gear reduction, and some electronics that allows it to move the motor to a position and maintain that position. They are used in making remote controlled model airplanes and cars. Usually they can only rotate a fixed amount, maybe 300 degrees. It sounds like your servos can rotate continually. This makes me think that your servo is different then what I consider a servo. You may mean an electric motor.
Does your servo look like this?
http://www2.towerhobbies.com/cgi-bin...&I=LXVW07&P=ML
This is important because making a simple electric motor move and making a servo move are very different.
Also, you're English is very good. It is not an easy language, and you manage to communicate your self better then some English speakers do! Keep up!
-Andy A.