Just imagine one of wheel 1 and 2 turned faster than the other. Then one could connect a motor to the slower one and gain speed for the same torque!!!
Another way to think about it: at contact points each wheel covers the same distance per unit of time. Then wheel 2 covers the same length of contact per unit of time and since it has the same diameter as wheel 1, it spins the same rate. ... assuming no slipping at contact points.
