Know this up front: GPS-based speed calculations lag. If you are ok with that, then this app delivers. I found it to be accurate (verified with my Garmin and vehicle speedometer) and the histogram is intersting, particularly over longer drives. Is just a speedometer, but it does what it advertises.
This app says that my Slash 4x4 goes 145MPH it only goes 35MPH.
The "Download" link for Speedometer Pro directs you to the iTunes AppStore, where you have to continue the download process.You must have an iTunes account to download the application. This download link may not be available in some countries.