I have found the solution and I will let it here for the people who might need it. As I have found out I think that the Vehicle Wheel Node doesn't actually rotate, but the supposed RPM that it should have can still be calculated as follows:
revolutions per minute (RPM) = speed in meters per minute / circumference of the wheel in meters
and
circumference of the wheel = radius * PI (aprox 3.14) * 2