I personally use 99.7Mhz. The signal still depend on the quality of the FM modulator itself. So far I already tested 4 brand, 2 is OK. Redfox and vztec one of the best. Another one is look similar to Redfox but has bad signal. The first ever tested is the one that use AAA battery. The signal OK but the sound quality is same as AM band radio (not FM quality at all).