For business purposes I have a Garmin GPS III Plus. It is a little large to carry all the time on the bike, so only every once in a while I snap tie it onto the handlebar to check out the accuracy of the speedometer. I assume that the Garmin will give a more accurate reading than the H-D speedometer. I find that the speedometer can be optimistic by as much as 10/11 mph the first start up of the day. It gradually settles down to run from 2.5 to 4 mph fast depending on speed. .............. BC
I can't specifically comment on the Quest 2 but I have used a few other of the Garmin GPS units over the years and they are all trouble free. FWIW, I have been looking at getting the Quest II myself. I already have the RAM mount for a much larger Garmin Streetpilot that I use on trips. So, when I get a smaller one for the bike, I'll only have to buy the cradle.