Originally answered on Quora on December 7 2016.

The conveyor belt essentially doubles the rotation speed of the wheels. In the case of the 747 the max rated tyre speed is 204 knots:

Exceeding Tire Speed Rating During Takeoff

And a typical takeoff speed is 155 knots:

Ask Us – Airliner Takeoff Speeds

In still air on the conveyor belt the wheels will be rotating at 2 x 155 knots = 310 knots, which is 106 knots faster than the max rated tyre speed. We need to reduce the ground speed to 204 knots / 2 = 102 knots, so we will need a 155 – 102 = 53 knot headwind to get our required 155 knot airspeed for takeoff. Presumably a person creating a runway-sized conveyor belt will have no problem sourcing a giant fan to create this headwind on demand…