do you want to have secure comms or make contact with others for help?.... you might get some "E" layer skip under certain conditions (600 or so miles)with the 27MHz CB... the 462MHz GMRS will have a lot of security channels if you want more secure comms, but both are sold as line of sight communications.. probably 1/2 mile or so reliably...I'm old school, and have a "peaked up" CB in the car with a tuned base loaded magnetic mount antenna, cigarette lighter power tap... I don't have any proof, but think you would more likely make a contact with a trucker (if near interstate) or old school REACT group with the CB, rather than the GMRS which I don't perceive as being "monitored" ... the number of GMRS units turned on in an "incident" would probably increase your chances