IMHO, the variable here is the vehicle mounted antenna. A hand held, whether it's a walkie-talkie, CB or HAM radio is limited by the antenna (and power, but I don't see that as an issue here). By vehicle mounting the antenna, you have a huge ground plane which will add gain to the antenna. I don't know what kind of wattage walkies put out, but comparing a 4W (legal limit) hand held CB with a 4W vehicle mounted CB, you will get better coverage with the vehicle mounted one due to the antenna.