Force on front and rear tires are in opposite direction?

[Fu-Kwun Hwang]

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The following applet simulated a front wheel drive car.
The engine provide power which drive the front tires rotate in the forward direction.
Due to friction force, front tires push ground backward,at the same time, ground push front tires forward. So the force on the front tires are in the forward direction.

However, the rear tires have to rotate with the front tires in the same rotation direction.
So the ground has to provide a back-ward push on the tire to make the rear tires rotate in the same direction as front tires.
If those two force are equal in magnitude but in oppositive direction, then, the net force on the car should be zero? Why the car can move forward?
What do you think about the above arguments?  What is wrong?

Think about it. 

Use mouse to turn on power check box, the car will star to move.
The drag force is assume to be in the form as Fb=-b*vx;
Red arrow shows the velocity. You can change the engine power with slider and turn on/off the power with checkbox.

Registed user can get files related to this applet for offline access.
Problem viewing java?Add http://www.phy.ntnu.edu.tw/ to exception site list
If java program did not show up, please download and install latest Java RUN TIME