const byte tachInterrupt = 1; // tach signal on interrupt 1 (digital pin 3)
byte engineCylinders = 8; // for tach calculation (pulses per revolution = 2 * cylinders / cycles)
byte engineCycles = 4; // for tach calculation
int refreshInterval = 750; // milliseconds between sensor updates
unsigned long previousMillis = 0;

volatile int RPMpulses = 0;

void setup()
{
  pinMode(tachPin, INPUT_PULLUP); // enable internal pullup for tach pin
  attachInterrupt(tachInterrupt, countRPM, FALLING);
  Serial.begin(9600);
}

void loop()
{
  if(millis() - previousMillis > refreshInterval)
  {
    previousMillis = millis();
    Serial.println(getRPM());
  }
}

// counts tach pulses
void countRPM()
{
  RPMpulses++;
}

// checks accumulated tach signal pulses and calculates engine speed
// returns engine speed in RPM
// Resolution: 30000 * engineCycles / refreshInterval / engineCylinders RPM (for default values = 20 RPM)
int getRPM()
{
  int RPM = int(RPMpulses * (60000.0 / float(refreshInterval)) * engineCycles / engineCylinders / 2.0 ); // calculate RPM
  RPMpulses = 0; // reset pulse count to 0
  RPM = min(9999, RPM); // don't return value larger than 9999
  return RPM;
}