Had nothing to do during the weekend and happen to see these apps on the appstore for HUD.
Thought to give it a try, well i have to say it looks pretty good, but not that real time of course due to delays from GPS.
Still it helps u to fulfil a little bit on car gadgets, that you have a HUD function haha.
On basic most of the basic HUD apps will display ur speed and also speed warnings when u go above speed that u have set.
Try it out, its free ! Works great at night but i think durring day it will not be so useful and ur phone will get hot too.
Below is a video :