Garmin Cadence Sensor vs. Garmin GPS

Are you saying in the same gear at the same rpm you're seeing a different reading?

Speed on the trainer is a meaningless number, by the way.

Same gear, same rpm (actually higher rpm inside). If anything I would think the number would be abnormally high - no hills, no wind, etc. to slow me down. 

And if speed is meaningless, then why does Garmin even bother with the technology? It's completely disheartening to think that I spent an hour sweating my backside off and I only "went" a measly 14 miles. 

Agreed with the statement above, speed is pointless. But I know that the resistance is much greater on the trainer than on the road. With that being said, if you want to see if your speed will go over 14.5 do not engage the resistance wheel. Mount the bike but leave the rear wheel free and spin if you want to see if you Garmin is working correctly in side. If you get into the big g????ear and sprint for a moment or two you should be able to easily get over 14.5.....

It's for those reasons that indoor training is based on heart rate, RPE, or power.

Here's what I don't get - same gear, same rpm means same number of rotations of the wheel. Because of the resistance of the trainer it may require more effort, but that doesn't change the above fact.

When you put the bike on the trainer is it possible you've moved the magnet slightly or sensor slightly and it's not recording 100% of the time? Can you check by slowly turning the pedal by hand and counting whike comparing it to the Garmin?

Is that right? In mm? Closer to 2100 would make more sense for a 700 wheel. What could be happening is that outside the 305 is using the GPS signal for speed. Then inside it uses the speedometer. The other guys are right in that a given cadence in a certain gear will give a certain speed, it should only be harder or easier to obtain that speed in different riding situations but they should always match up.

Yup.  And Sheldon Brown lists it as 2097, so your wheel diameter is only one meter too small

Circumference, not diameter.

I reject your reality and substitute my own!

err... thanks  That's what I get for typing faster than I can think  

When in doubt, RTFM.

Wow, this thread got snippy in a hurry.... Typo on my part in the OP, sorry. The 305 bike profile was set to 2096, not 1096. I have now corrected it to 2097 but 1 mm is hardly going to cause a variance of 3-4 mph. 

I always turn off the GPS when I'm inside. I can't get a signal in my house or in the gym and as mentioned above, it just keeps cycling to try and reach the satellite. 

I'm now beginning to wonder if something is up with my Garmin because I did as Dub_Z suggested and removed the trainer resistance. I got the speed up over 35mph (man wouldn't that be great out in the real world!). When I put the trainer tension back on the wheel (same position), I was getting readouts of about 17.5mph - which is more where I should be. 

I ride again tomorrow, so I'll just see what happens then. Thanks all for your input! 

Speed on the trainer doesn't really tell me much.  I can turn the resistance down to nothing and do 30 mph at no effort OR crank up the resistance and go 10 mph and be sweating bullets.

I know my average speed outdoors at a given HR.  If I'm I average zone 2 outside, that's about 16 mph on my rolling route.  If I'm in Z3, it's about 17-18.

When I'm on the trainer, I look at my average HR and keep it in the zone I want and credit myself those miles in my logs.  So if I bike an hour in zone 2, I log 16 miles.  I may be a mile off, but does it really matter?  All your body knows is effort and time.  It could care less about speed or distance - you heart and lungs don't know the difference between 10 mph or 30 mph, just that it's beating at 156 bpm for 60 minutes.

How did you generate that comparison?  From your posts, I believe you pedaled at the same perceived effort and find that you are traveling at 1/2 the speed you expected.  That is normal.  The trainer acts like an effective hill.  It applies a resistance.  If you turn the crank at the same rate (cadence) in the same gearing, the wheel will turn at the same rate (speed) no matter where you are.  Depending on how much resistance (trainer, aero drag, or hills) you have will dictate how much effort (power) you need to turn the wheel at that rate.  That is why speed is irrelevant on a trainer.  You are fighting more than just the effort to turn the wheel and that has no correlation to the "real" world.