Unfortunately there is no easy answer to this question.

All Cobra CB radios transmit using the maximum FCC allowable power output of four watts.  There are two main factors that will affect the range of your transmissions:

RADIATED POWER

Although the radio transmits with 4 watts of power, the performance will be affected by how efficiently the antenna radiates this power.  Contributing factors include the following:

The condition and location of the antenna (higher is better)The physical length of the antenna (longer is better)The proper matching of the standing wave ratio or SWR.  (For more information on setting the SWR, please refer to the relevant FAQ)

ENVIRONMENTAL CONDITIONS

Tall buildings, trees, hills, and interference from other CB radios all limit the effective range of a CB radio.  Certain atmospheric conditions (such as sunspot activity) can also affect the range for CBs.

If we were to eliminate all negative environmental factors, a properly set up base station could communicate between 10 to 15 miles, a mobile unit 5 to 7 miles, and a hand held unit approximately 2 miles with the only variable being the type of antenna used.  Unfortunately, these optimum conditions rarely exist and the range of your unit will typically be less depending on the conditions under which you are operating.

The easiest way to maximize your range is to use the longest antenna possible, and make sure that the SWR is properly adjusted.