Angular Speed with arduino

Projects General Guidance
14 
 delay(12,5);

This is bad syntax. delay() takes an unsigned long.

If you want to take measurements every 12.5 ms, replace the delay() and use a timer with micros() like the one used in the "blink without delay" example in the ide. See also Robin2's tutorial on millis() timers. Demonstration code for several things at the same time - Project Guidance - Arduino Forum You will use micros() instead of millis() for better precision.

if (micros() -timeOfLastReading >= 12500)
{
timeOfLastReading +=12500;

//take a reading and calculate
}