Gear joints are one of the real specialities of Box2D.
By creating gear joints between two bodies you can actually simulate a real world gear. For creating a gear joint you need two bodies connected by a revolute or prismatic joint.
The below code creates two bodies in the shape of a circle which is actually attached to the ground. however you can also create joints without joining it with the ground. Here you have to make a ground with possible four fixtures or no fixtures simply creating a body.
b2Body* body1 = circle1;
b2Vec2 pos1 = circle1 -> GetPosition();//get position of first body.....
b2Body* body2 = circle2;
b2Vec2 pos2 = circle2 -> GetPosition(); // get position of second body....
revJointDef.Initialize(ground, circle1, pos1);
b2RevoluteJointDef revJointDef2;
revJointDef2.Initialize(ground, body2, pos2)//revolute joint is initialized
joint1 = world->CreateJoint(&revJointDef;); //First joint.....
revJointDef2.maxMotorTorque = 20000.000000f; //connect a motor to the joint2
revJointDef2.motorSpeed = 120.000000f / 60; //set the rotation speed.....
revJointDef2.enableMotor = true; // enable the motor.....
joint2 = world->CreateJoint(&revJointDef2;); //Second joint.....
gearJointDef.bodyA = body1;
gearJointDef.bodyA = body2;
gearJointDef.joint1 = joint1;
gearJointDef.joint2 = joint2;
gearJointDef.collideConnected = true;
gearJointDef.ratio = circleShape2.m_radius/circleShape1.m_radius;
world->CreateJoint(&gearJointDef;); // this line creates the joint.
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