I have a Garmin Forerunner 301 XT watch which gives me speed information. I wanted to correct my cadence so I was thinking of buying a cadence monitor. But I don't see the point of buying that extra speed sensor. Am I missing something here ?

Another question:
I have two options for a cadence monitor:

- Garmin Bike cadence sensor
- older Garmin GSC 10 speed sensor

From what I found out, the first one is all self contained and has no external parts. But, I was wondering if anyone here has used either of these and would recommend getting one over the other ?

Speed data taken by GPS is not nearly as reliable/accurate as speed date taken from the wheel rotation. If you can afford a speed sensor to measure by the frequency of wheel rotation, I recommend that.

I have the GSC 10. If you don't want the speed info then just get the new cadence sensor. Easier set up of only 1 part.
It is compatible with your 310 XT.
More than you will ever need to know on this subject - Read this:
A look at Garmin?s new ANT+ Speed & Cadence magnet-less sensors | DC Rainmaker

I hear this a lot, but do you know why? And how big are the variants?

One of my bikes doesn't have the speed sensor, and I ride to work with it some times. The distance at the end is pretty much the same (to within 0.1 mile) as the one speed sensor. So I am assuming the GPS is measuring distance ok.

In theory, yeah. In practice, I found the opposite to be true over the course of a long ride. Garmin products are buggy, the sensor would just cut out and my speed would be zero for blocks at a time. I turned it off and just use GPS for speed.

Also, I'd like to add to this. I used to run with this watch and the distance was pretty accurate. Not sure about the accuracy of speed but it seemed in line with what the other folks reported, with whom I used to run with.

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.

Things may have changed, but in the past GPS receivers often lost touch with the satellite especially if kept in a pocket. You never got permanently lost but the exact route you had ridden might not have been recorded. Also there is an error in every GPS estimation of position. Used to be maybe as much as 50 feet. Now that everyone has access to the military system, it is much smaller. Still there is imperfect precision. So I have always thought use GPS for mapping and sensors for recording bike data. Perhaps that viewpoint is already obsolete.

+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.

Both sensors that you linked will provide both speed and cadence. As mentioned, the speed sensor is more accurate than GPS speed, but not sure how important that is to you if you find a cadence only sensor. I've used the Garmin GSC 10 for years and it performed flawlessly. I've heard good things about the newer Garmin, but not sure if its worth the extra $ just to eliminate the magnet. Others using it may chime in.

No use, I am using the Lemond Revolution.

The US government stopped the use of Selective Availability (SA) back in the 2000. They were intentionally degrading the signal for defense reasons and accuracy was less than 50 ft. The accuracy now is based mostly on the antennae used. The commercial grade devices which include the bike and handheld computers are good to about 3 meters horizontal accuracy. Pretty good in my opinion, but vertical accuracy is poor. The differential grade devices are accurate under 1 meter, have much better antennas and are used for surveying, mapping, etc.

Threads like this remind me why I never gave up wired systems. I can get 10 years out of a battery, readout is faster, accuracy is at least as good if not better, smaller/lighter, and cheaper to boot.

This post reminds me why I never got rid of the wired telephone that still sits in my closet for all these years. No need for battery. Plug in, lift the handset, and voila, dial-tone.

...but how are the aerodynamics?

I just installed the new Garmin magnetless sensors this weekend, piece of cake and you don't have to worry about them getting out of alignment. Paired it with an Edge 500 I found refurbished on Amazon.

Probably come out ahead there too. Sure you get some drag off the wires, but the head unit, mountings, and sensors are all smaller...

Been running the GSC10 with a Garmin 800 for 4 years. Had to replace the battery once. Works like a charm, no problems. This is on a tandem. Signal is received fine in both the stoker and captain's positions. I have the 800 set for automatic tire size, so it uses GPS data to calculate tire circumference. That works great, too. I'm always right on the distance when I follow courses or cue sheets. Of course you have to pair the sensor with the Garmin head unit, but that is explained in the instructions which come with the sensor and is easy to do.

But is it sexy tho