Advantage of speed sensor?

From what I understand, it's most important for MTB riding, where GPS reception may be poor, and/or tight switchbacks on the trail might result in the GPS "cutting corners" and under-calculating distance.

Is the speed sensor more accurate than the gps? Does it update the speed quicker and give quicker speed readings?

Tom

There are position errors with GPS that tend to cancel out over distance. For most road biking the overall distance and average speed for a ride is very accurate with GPS. But over a short distance the position errors can cause fluctuations in speed, the auto-pause to come on when going slow especially under tree cover and the gradient readout to be less reliable. I don't think the speed readout is any quicker with the sensor. As previously mentioned for mountain biking the sensor may provide more overall accuracy.

Yeah, honestly the only real reason I have a speed sensor on my road bike is that at the time, the combo speed/cadence sensor was all they had... I don't think it's needed. One caveat, however, is that there are reports that the new style cadence-only sensor is a bit slow in responding to changes (like if you are coasting, and then start pedaling again... the cadence doesn't register right away, or so I've read). This may or may not be a problem for you (I personally don't think it would bother me).

One more aspect to consider: indoor training. GPS is useless on stationary trainer / rollers.

Yeah, but since your speed is zero, so is a speed sensor. Using one only makes any sense at all if you're riding rollers, and even there the opposing forces you feel on the road just aren't there. Speed is a useless metric indoors.

actual speed on road can sometimes show low with GPS

My commute bike doesn't have a speed sensor - I just use a computer with GPS (e.g. Bryton rider 40 or Fenix 3), my road bike does. What I notice is that while on the commute bike in heavily tree-lined sections my speed can show several kph low from what I expect. The total distance (and hence also the AVS) seems to always come out OK because the roads are not very bendy. (Note that AVS always seems to be just total distance divided by ride time).

Biggest benefit is that in situations where you lose GPS connection the speed sensor will continue counting your miles. My first imperial century I got caught up in some pretty severe cloud/overhead. GPS signal was completely lost. I had the route in my Garmin and it stopped providing me turns. That's to be expected without GPS. But I pulled up the cue sheet which gave me the distances and then I had to try to guess when I was coming up on a turn. Had I had a speed sensor I would've known that I had traveled X miles and my turn was coming up.

The other thing is that in cases where the GPS isn't a strong data, the speed sensor will carry on. In this situation you may get GPS signal every once in a while but it'll have bad data. Speed sensor has good data.

As far as accuracy in GOOD conditions...I'm pretty comfortable with relying on the GPS. Especially the new GLOSNAS (sp?) which is much more accurate than my edge 500. I currently don't use a speed sensor on my Time Trial bike because my training and race wheels both have too tiny of a wheel hub to support the speed sensor (both front and back wheels). I haven't really had any issues of losing GPS connection, but I do notice the battery drains more quickly when using the GPS+GLOSNAS (sp) over having the speed sensor.

TL;DR:
Speed Sensor
Pros: always counting your miles, i don't think it uses as much battery (i don't have hard proof on this claim)
Cons: costs money, may not fit all wheel hubs (it'll fit most stock wheels tho).

Yeah, but since your speed is zero, so is a speed sensor. Using one only makes any sense at all if you're riding rollers, and even there the opposing forces you feel on the road just aren't there. Speed is a useless metric indoors.

I'd guess that most people want to know their speed/distance on an indoor trainer. The speed sensor gives you that. And it's pretty much mandatory for Zwift (for those so inclined).

If you are concerned about monitoring your current speed, then the speed sensor is needed. Without the sensor, the current speed will fluctuate wildly especially if you are in a area with tree coverage or tall buildings. I've watched the speed go from 8mph to 16mph when my sensor battery fails.

Another consideration is if you are riding in a mountainous area. Although very small, the distance will be not be calculated as accurately without the sensor because using GPS alone will not take into consideration the vertical climb and will consider the mountain as flat. Very small difference but something to think about.