Suppose you have an electric-assist motor that can propel you at 15 mph by itself on level ground. You would like to ride at 25 mph so you start pedaling. Will your speed be equal to the sum of the forces or will the motor actually start to be a drag at some point?

If you want to ride (and pedal) at 25 mph do you need to start with a motor that will turn the wheels unloaded at 25 mph or more?