How far(long) the transmitter cover?

The transmission range depends on many factors. The true distance is based on the sensitivity of the receiver, antenna of the receiver, and the building and other obstructions, which are between the transmitter and receiver. And the distance will much more in countryside. 

EXAMPLE 5W:

I have a USA customer use 5W fm transmitter with GP antenna in his hometown ,and he test it with a car ,it cover 10km(6.21mile).

I test the 5W fm transmitter with GP antenna in my hometown ,it cover about 2km(1.24mile).

I test the 5W fm transmitter with GP antenna in Guangzhou city ,it cover about only 300meter(984ft).