gistfile1.js

body1 = new Body(physics, { color:"red", x: 15, y: 12 }).body;
body2 = new Body(physics, { image: img, x: 25, y: 12 }).body;

var def1 = new Box2D.Dynamics.Joints.b2RevoluteJointDef();
def1.Initialize(physics.world.GetGroundBody(),
                body1,
                body1.GetWorldCenter());
var joint1 = physics.world.CreateJoint(def1);

var def2 = new Box2D.Dynamics.Joints.b2RevoluteJointDef();
def2.Initialize(physics.world.GetGroundBody(),
                body2,
                body2.GetWorldCenter());
var joint2 = physics.world.CreateJoint(def2);

def = new Box2D.Dynamics.Joints.b2GearJointDef();

def.bodyA = body1;
def.bodyB = body2;

def.joint1 = joint1;
def.joint2 = joint2;
def.ratio = 2;
var joint = world.CreateJoint(def);