Ive found this glitch...
This glitch will "turn off" your speedometer
First find a place where it can auto-reset your car
(ocean,sea,shoreline recommended)
Second drive to the place and right before it resets itself push the start button
Third you know that sound when you exit the start screen yea...you should hear it
Thats it i guess.:D
this will not "turn it off" forever,to turn it back on you just press start and that's it.
Ive only done this on hardcore mode so I'm not sure if you do it regularly
Note:This might take a whilepress start one t so be patient
I recommenced that you press start when you are flying into the water and then the one right b4 you hit the water