This app was mentioned in 7 comments, with an average of 1.71 upvotes
Try https://play.google.com/store/apps/details?id=it.pirelli.diablosuperbiker
(https://play.google.com/store/apps/details?id=it.pirelli.diablosuperbikerus did not work for me - looks like it's a US version)
It's on Google Play but when I went to install it (from a browser, not the phone) it said I had no eligible device. :( HTC One isn't an eligible device??
I don't have any experience with other models, but I used this program to log the speed on my Kawasaki Ninja 250. The speedo read about 10% faster than the GPS log. I was near sea level going at a constant speed on a straight road for several minutes, so the GPS speed is really accurate.
I would guess that the speedo tolerance is regulated at something like -0% +something%, to ensure that no ends up going faster than their speedo says they are.
I have heard rumors that some motorcycle manufacturers purposefully make their speedos err significantly on the positive side, to make it seem like their bikes are faster.
https://play.google.com/store/apps/details?id=it.pirelli.diablosuperbiker
It's for Android too but not in America apparently
Use this sometimes it is pretty good
https://play.google.com/store/apps/details?id=it.pirelli.diablosuperbiker&hl=en_GB
It's an app called Diablo super biker https://play.google.com/store/apps/details?id=it.pirelli.diablosuperbiker