Honestly, i use the elastic strap from a $3 armband to hold my iPhone 5 to the headrest of the passenger seat. Works fine.
EDIT* I also checked the app store for some dashcam apps, and there is one called iSymDVR that is currently free. It does not have on screen speed but is only in version 1.10 and they say in the description that 1.11 will have on screen speed display. Its only free until Oct 20, I figure get it now for free and when they add speed, you'll get it as an upgrade. This could be a huge cover your ass if you are in an accident that is not your fault, or are pinched for speeding when you were not speeding.