Tachometer arduino

 volatile byte rpmcount;
 unsigned int rpm;
 unsigned long timeold;
 void setup()
 {
   Serial.begin(9600);
   attachInterrupt(A5, rpm_fun, RISING);
   rpmcount = 0;
   rpm = 0;
   timeold = 0;
 }
 void loop()
 {
   if (rpmcount >= 2) {
     //Update RPM every 20 counts, increase this for better RPM resolution,
     //decrease for faster update
     rpm = 30*1000/(millis() - timeold)*rpmcount;
     timeold = millis();
     rpmcount = 0;

     Serial.println(rpm);
     rpm=0;
     delay(100);
   }
 }
 void rpm_fun()
 {
   rpmcount++;
   //Each rotation, this interrupt function is run twice
 }