I have a Magellan unit. I don't think it's meant to be on a motorcycle, but I've had it on my 600 before. You can't read it in sunlight. I'll stick the GPS on the 919 to double check. I am really looking for a starting point.
Go to the drag strip and cruise through the speed trap at 100mph indicated on the speedo. This will give you error %. Several threads have documented the stock error at 8% the speedo-healer corrects for speedo error at 5.5% so if you increase the recommended correction factor by 2.5% it should be real close. I have checked my speedo at the track and going through the traps at 100 indicated results in a measured speed of 98.5. I can live with a 1.5% error.