An antenna for your car... Tricky dickies has a 7dBi antenna with a magnetic base that does the job nicely... if you want something more permanent though, borg has an antenna designed just for the task..
Those high power wireless adapters are fine, but you should also use the best possible antenna. You can buy some good ones out there, but if you can measure cut, and do some soldering, I offer the many long range wifi antennas at my website.
Taylor made for people who need wireless coverage over large areas.