I've had no luck with FM transmitters. I live in the boulder/denver area and cannot for the life of me find a frequency that doesn't produce static or interference.
I bought all sorts of transmitters and finally gave up and went with an FM Modulator.
A little more involved of an installation (if you're handy or have installed car stereos before it's very easy) but far superior performance.
DO Not, I Repeat DO Not buy any trasmitter made by Griffin or Belkin. Buy a Monster Cable FM Transmitter. It works great. I have had all the others and they do not work very well and I a Belkin one where the iPod sat in and into your car cigarette lighter abnd I fell apart. I have had the Monster brand transmitter for two years now and is still working well. I have used it all over North America and seems to do very well. Even in FM crowded cities such as LA.