It's pretty simple, it just shows the current speed if the hero is driving vehicle.
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 and unpack the .asi file and .ini config and put it into the root directory of GTA5 (where the GTA5.exe is located).
Change it's config as it's described in the speedometer.ini file Use slash [ / ] key to change the way the data is displayed: KPH, MPH, Hide speedometer
Problems:
There is a problem with positioning, it's because of the Safezone size setting. I haven't found a good solution, so I reset my settings to defaults, and positioned the Speedometer box to look best for that settings.