Garmin gave me a VAM on one ride of 1100, which I was skeptical of from the start. I think what happened was that I was riding a MUP along the Santa Ana River (SART), which is pretty flat, except where is crosses under major streets. Being Clyde, I hit it going down into the underpasses so I can be carrying enough inertia to sail out the other side at a reasonable speed, often as high as 20mph. I'm guessing some of these are 10%, or there abouts. Even though the total ride is pretty flat, it was reporting max VAM, which I was hitting for only100-200 ft horizontal, with the help of Issac Newton.
My fastest legitimate VAM was 416 on Torrey Pines Grade in San Diego.
http://app.strava.com/activities/12313984#210491526
Age-56
Wt232 (240 at time of climb)
I'm hoping to be below 220 by Labor Day, when I will hit Torrey again.