Hi, <montra@>
an SSB CB on 12 Watts will give you 20-30 Ks if you can set up the Base antenna preferably as high and clear as possible with decent coaxial cable to a Slim JIM antenna ( More info on this site) which you can make yourself which has a low angle of radiation which is better if it is positioned on top of a hill, or a 1/4 wave verticle if you just want to buy one.
Some might say a .64 Wave ground plane antenna is also an advantage.
Obviously the Mobile would need to be optimised as well with a decent whip and an SSB CB.
Although 27Mhz CB can travel this distance on "Ground Wave" conditions need to be good to achieve reliable comms.
You could consider UHF CB at 476Mhz through local repeaters if available also depending on conditions and the best thing to do would be talk to a local that you see on the road /street with a CB antenna that would be aware of local situations that could assist you.
This is by no means a definitive answer as its impossible to determine from the limited information you have supplied.
