An 11-mile contact is good, but it is possible to do much better, even without using twin antennas or SSB. In 1979 I had a confirmed 32-mile CB contact, over flat ground between Firebaugh, CA. and I-5 -- I was slightly higher, probably about 50', than the station I was talking to -- using a single 4' Firestick and a 4-watt 23-channel radio Panasonic radio. I was driving a 72 Ford Pinto wagon at the time. The base station was also a ham, and sent me a QSL card with his phone number. Using a map, we measured the distance at last contact as 32 miles. The secret was a SWR of 1. 1:1; as I remember, we were on channel 19.