Originally Posted by
TheLifeOfBryan
(I believe that the full cadence setup measures speed directly and shows that on the LCD, but I'm not 100% certain.)
That is correct. The Garmin speed/cadence detector uses a magnet on the wheel and another on the crank. It is as immediate as any other cyclometer. GPS-measured speed is quite accurate but it takes a while to get enough detail and the satellites have to be visible. I'd guess that GPS speed is more accurate at higher speeds (and longer distances). The GPS is probably not so good at measuring your speed doing slow donuts in the parking lot!
Originally Posted by
TheLifeOfBryan
So if you're leading a paceline and need to maintain a dead level speed, a bike computer is essential.
Good point. There has to be some reason for the added expense of the seemingly "reduntant" speed measurement!