Is the speed sensor basically used only when riding a trainer? What and when would it be applicable for actual riding on the road?

The reason I ask is that it would be cheaper for me to buy the stand alone 520 unit and then buy the HRM separately than to buy the bundle. I don't see myself the speed sensor if it's for indoor riding only.

If you have a speed or speed/cadence sensor attached, the Garmin will use it to determine speed and distance over the GPS. It also responds quicker than a GPS when refreshing the display. Like any other bike computer, you have to enter the wheel diameter to calculate the correct speed and distance. If you ride in an area where GPS signals are prone to getting lost, you'll definitely want the sensor so you'll have speed and distance. And yes, it can be used indoors on a trainer as well.

Your Garmin can measure your speed and the distance you travel by:
It wants them in that order. The sensor counts how many times your wheel turns, so it's obviously going to be more accurate under normal circumstances. Debatable whether it's more or less accurate on a trainer (you haven't moved the bike at all so it's zero miles, but you've worn your chain and tires). I'm not sure if you're looking at the one that uses magnets or a motion sensor; if the magnets aren't lined up properly it's going to be much less accurate than GPS.

I use GPS for speed and distance. If I go out and do a 40 mile ride, but the computer only records 39.9 miles, it won't affect my life in any meaningful way.

Thank you for your help. That's exactly what I needed to know.

I use one with my Edge 500. I used to loose GPS signal under roadside trees cover. The speed sensor eliminates that. Although I guess if that's the case I should have bought a wireless non-Garmin computer for 1/4th the price.

That's exactly why I just bought the garmin speed and cadence bundle (for my back-up trainer bike).

I found that without the speed sensor, the GPS would momentarily lose signal when I was standing and climbing, with my body obscuring the Edge 500's line of sight to the sky. I would get random auto-pauses and speed drops/spikes.

I've also found that outdoors, the Garmin gets confused with the speed sensor on the rear wheel, so I have to run it on the front hub. But for whatever reasons, it has zero issues indoors on the trainer.

I've gotten use to mine over the past couple of weekend rides and I have to say I really like this thing. I'm not using it to it's full capacity but it's nice to know there's room to grow. I have also had zero problems.

for those interested,, the Motorola speed/cadence sensors on 3bay are compatible
with Garmin units. and are a fraction of the cost of garmin gsc10 sensors. ive had
no issues with mine. I have no affiliation with any of the sellers.

I live in the City of Urban Canyons, so the GPS track often has me riding like a drunken lunatic, crashing through buildings a block or two from where I am. So I use a speed sensor.