This mod I created a few days ago (right after the ScriptHookV was released). It's pretty simple, it just shows the current speed if the hero is driving vehicle (or other transport).

By default it shows speed in KPH (kilometres per hour), but you can change this using slash [/] key.

Installing and using:

You need to install ScriptHookV, so follow this instructions.
Download the .asi file and put it into the root directory of GTA5 (where the GTA5.exe is located).
Use slash [ / ] key to change the way the data is displayed: KPH, MPH, Hide speedometer