Tunear the measurer of kilometros or speedometers

The speedometers are one of the most important parts of the vehicle, so important that we must try that is always in perfect state, since is what all the information of the vehicle transmits until us.
It has several spheres where one generally visualizes the kilometers per hour to which the vehicle moves (speedometer), another one where it indicates the RPM of the motor (tachometer), and two smaller than they indicate the temperature and the amount of remaining fuel.
There are them more modern than they are digital, instead of the spheres for the fuel and the temperature; but the normal thing is to always find them like minimum with speed and revolutions with needle.
What can be modified of the speedometers?
color of the inner spheres.
frontal part that is bordering the spheres.
needles.
illumination of the needles and spheres.
As a rule the modification of this one part is not legal since the kilometrage can be changed manually.
Types of Speedometers.
The types of speedometers that there are are:

. Digital
. Mechanics
. Electronic