Originally Posted by
Steve B.
Learn my on the need for a speed sensor ?.
I kept my Cateye sensors on my bike when I added a Garmin 810 and do notice that there's a slight lag on the 810, maybe 3-5 seconds at best ?, of current speed. Distance seems to be off about a mile in 100 and the auto-stop/start is different as well, so I guess ride distance/time/avg. is different, but somewhat a "so what".
So why do these exist ?.
They don't add miles from GPS drift, work in areas with spotty GPS signals, and work on trainers.
While speed is meaningless on trainers, it's nice to know fast different tires and chains wear out so you know what to buy next. I found that I got about half the life out of GP4Seasons compared to Gatorskins; and GP4000s which are even faster didn't wear out any quicker than Gatorskins. Without a reasonable tire-distance-traveled estimate you couldn't know that.
They may also keep your computer map display pointed in the right direction when you stop with a computer lacking a magnetic compass. While GPS drift can trick the computer into thinking you're moving and changing directions, 0 speed from a sensor should let the computer assume you're still facing the same direction.
That can be a big deal when you're 20 hours into a ride (you don't think as well at that point), it's after midnight (you may be unable to read street signs without a helmet mounted light, and aren't wearing one because that's tiring potentially leading to Shermer's neck), and you're in an unfamiliar location.