Garmin distance issues

[TomE.]

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.

http://connect.garmin.com/activity/197624840

[c0nsumer]

You need to ride roughly 1/4 to 1/2 mile in a straight line to get auto-calibration to occur. The unit will pop up a message that says "Calibration Complete" or something like that when it happens. Note that the message only appears for a few seconds so it's easy to miss.

I personally ride a straight residential street near my house after swapping rear tires/wheels to force this to happen. Also, switching it to manual wheel size than back to automatic will force a recalibration. Just be sure you ride in a sufficiently straight line. I don't think the parking lot to trailhead path at Maybury would work.

[TomE.]

I figured the calibration was off, but even when I put in the correct wheel size manually it came up a mile short. I'll try calibrating on a straight city street, see if it comes close to my roll out measurement.