Originally Posted by
WhyFi
You'll want the speed sensor for those wintry days when the temps dip in to the 50s and you're forced to stay inside on the trainer.
+1 only reason I'd bother with a speed sensor. Over the length of a ride I've found GPS vs speed sensor speeds to be within .1 MPH. That is well within my comfort zone for accuracy. However, I'm not trying to set a Guinness Book of World Records speed record so YMMV.