This app was mentioned in 2 comments, with an average of 1.50 upvotes
It used to just be some guy's personal project and then Predator bought it, so now it's the predator break speed app. It's pretty cool but apparently not that reliable. (two phones will give different numbers running the same app). Still, it's at least close.
iphone version: https://itunes.apple.com/us/app/break-speed/id371892578?mt=8
android version: https://play.google.com/store/apps/details?id=com.cymobilesoftware.breakspeedpro&hl=en