Garmin 310xt/GSC 10 pairing accuracy

Trainer speed has nothing to do with outside speed. A 100 pound person going 20 mph (lets make up 150 watts) outside needs a lot less power than a 200 pound person going 20 mph (lets make up 250 watts). Now switch it around...

Lets say at 15 mph you need to hit 150 watts on your trainer. This is going to be a workout for a 100 pound person while the 200 pound person this may be an easy ride since they can ride at a much higher power outdoors. Basically, ignore the speed on the trainer. It's just an FYI thing unless you sort of know that x mph is normally your interval zone (based upon past experience using your HR). My battery in my GSC went out a while ago and I've been too lazy to change it since I train with power and my PM sends out cadence too. Speed means nothing if you're stationary and not going anywhere

What kind of trainer ?

I think it is a Trac one? It is the Performance Bike brand, orange and white, a fluid trainer.

I don't know about that one.

I agree with the comment that distance and speed are not good metrics on the trainer. HR and Power are better

However, there are some trainers, like the Kurt Kinetic that do have a pretty good power to speed correlation. It's in line with a very flat, no wind, road bike position, but it was made to mimick real world power vs speed.

As a matter of fact people that don't have power can use speed as a proxy on these trainers.

They simply require proper setup of the wheel size in the 310xt and a correct setting of the tension

It's not perfect but speed on these units is somewhat comparable to flat, no wind riding.

Sure, speed is a great substitute for power on a trainer with the right precautions (same tire pressure, trainer resistance), but that is not really the issue.

As mentioned above, the same effort (power) on the road may not produce the same speed on the trainer, and that is fine. Just normalize those variables mentioned above, and then you can (somewhat) compare trainer sessions by the speed and distance.

If it really bothers you, you can tweak the trainer resistance until it your bike computer matches your perceived speed. Just be aware that there needs to be enough (and not too much) tension of the roller on the tire!

Have you tried using traineroad and sufferfest? Traineroad calculates your power based on your bike trainer and the speed sensor. I just got the cyclops fluid 2 trainer and garmin speed sensor