I have the Garmin 800 with a GSC10 sensor, and I cannot for the life of me get the unit to display the correct distance. I was at Maybury last week and did two laps with two different setups, and it is still out of whack. I know one lap at Maybury is going to be in the 6.35 mile range, based on my older-but-$400-cheaper-than-the-Garmin Speed Zone computer. On the first lap, I set the 800 for auto calibration of the wheel size and smart recording of data. I rode from the parking lot to the trailhead with the units on to get a decent calibration of the wheel size. The unit claimed it found the sensor. The lap came in at 4.5 miles. the wheel had been calibrated at 2174. I changed the wheel size setting to "custom" -> 2280 (based on rollout measurement done earlier in the day) and changed the data recording to 1 second intervals, and the lap came in at 5.25 miles. Laps at Novi come out in the 8-9 mile range, nothing close to where it supposed to be.
I suppose I can keep changing the wheel size until the distance is more accurate, but I think I'd run into exponential issues on different trails. I've been scouring user groups for a fix for this since the GSC10 unit only came with less than helpful guide on how to strap it to your chainstay, but have come up fairly empty handed. Any tips or pointers would be very appreciated, as I'm getting kinda frustrated at the cost-to-product satisfaction ratio of my Garmin.
Edit: here is data from the Garmin on a ride earlier in the week at Maybury. I did the "B" loop, with splits at the B loop both times I passed it. Probably no help, but it shows how far off the mileage is.