Personally I use the digihud speedometer app, and funny enough velcro is definitely strong enough, I used to use my primary cell phone even as my speedometer as well and I used an old belt clip holder by otterbox for a Samsung s4, and I had an HTC m8. The case in the phone had the soft part of the velcro while the belt clip which I zip tied to the bars had the rough part, and I used the actual velcro brand which has a heavy duty/industrial strength velcro with an adhesive back, it's supposed to support 10 pounds so a cellphone is nothing.
It worked, and still would work if I put the back on the bike, the velcro stood up to about 6 months of use and never felt like it would drop the phone, in fact I had a bit of chain zip tied to the actual belt clip since I was more worried about the zip ties failing than the velcro.
Check out the app it's kinda nifty, and they have an onscreen window that can be moved around too, so Google maps could be used in the background for navigation while a small window displays your speed.