TL;DR
My garmin is messing up, speed not reading as fast as I'm actually going...more like 4 mph too slow

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Rode today and felt really really good...one problem...My garmin said I was going really really slow. It was a group ride and my legs were fresh but I would get up for my pull, look down to make sure I wasn't dropping the pace and saw that my garmin said I was doing 17mph....we usually move at about 22-24 in the sections I pulled. Naturally I kept pushing harder to get up to speed but it just burned me out faster which made me feel like, maybe I wasn't feeling as good as I thought I was.

when I uploaded to garminconnect it said I had a 16.9 avg but I had 2 points where I hit 42mph...(no clue where that couldve been lol) and the data was all over the place

Strava auto corrected my avg to a 21.1 (I didn't stop the garmin since i thought I did horrible) which sounds closer to right. The others I was with average 22.1 on the ride which seemed about right, it was super windy and it was a smaller group so we weren't as fast for as long as we usually are.

The question is, is it my battery, the sensor, the garmin itself? has anybody else had this problem? I plan on working on it tomorrow.

A couple years ago I had the same problem. Mine was because the wheel magnet had slightly slipped. Check first to make sure it's lined up with the line on the GSC-10. If that's not it, it's something else. But I bet that's it.

perfect...lets hope so. I can't imagine why it would have moved, although my friend kept my bike in his element for a couple of days and something may have rubbed it putting it in or taking it out. I'll update tomorrow.

I second a possible wheel magnet issue, a couple of months ago when i started out on my ride i normally spin and hold a 15 mph speed the first couple miles to get the legs warmed up. I looked down at my garmin and was recording speeds of 30+ mph. Obviously i knew their was an issue so i hopped off the bike and fiddled with the magnet. That fixed it and all was fine again.

I ride mine without the wheel magnet.
Figuring the GPS would be more accurate anyway...no?

I'd say its time to replace the battery in the speed/cadence sensor.

What's your wheel circumference set to? AUTO or a fixed number?

I ride without the sensor anymore. I just use the GPS. It is very accurate. If you need to use the sensor then swap batteries, and make sure you have the wheel size set to "Auto" so that the GPS can still autocorrect it if it gets massive out of calibration.

reset my cadence sensor and it worked fine, mid ride it messed up again and started giving me random readings, hit 94.1mph at one point...

fixed it mid ride and managed to finish without more than 2 false readings....we figured when it does mess up its reading about 5mph too slow...

I'll replace the sensor battery first tomorrow and hope that works...I need the cadence monitor because its something I'm still working on perfecting (or as close as i can get)

p.s. it was set to auto so i ruled that out