I have the 305 and the speed sensor, and have specified tire size in the settings of the 305 (something that may help the speed reliability) but I still find the garmin to be off compared to the powertap or cat eye computer... max speed is usually 1mph+ slower than the others, and it gets funny in certain areas (I assume because its still using gps data) its spot on indoors, however, when I was on the trainer, and the average speed is dead on, so I dont care too much about the rest. just annoying.