One problem related to speed sensor has been reported for the 2009 Mazda CX9. The most recently reported issues are listed below.
The tachometer of the car sticks in high revs (1500-2500),engine on, car in neutral, foot off the gas pedal, if the car is moving over 20-25mph (either by free falling a hill or being pulled) the tachometer varies the rpm reading to match speed just as if the car would be in 'd' position. I think the ecu has a glitch of some sort that ignores the gas pedal and the shift position if the car is moving. Taken to the dealership and they said car is working as designed, that the car will keep rpms to match speed in to avoid a engine slowdown or something similar. My concern is that the ecu should behave as expected so should the tachometer read 600-800 rpm when idling, regardless of the speed of the car if the transmission is in neutral, if not at any time the ecu may decide it should rev up the engine if the speed sensor goes bad.