Arduino rpm teller

int ledPin = 13;
volatile byte rpmcount;
unsigned int rpm;
unsigned long timeold;


void rpm_fun() {
   rpmcount++;
}

void setup() {
   attachInterrupt(0, rpm_fun, FALLING);

   pinMode(ledPin, OUTPUT);
   digitalWrite(ledPin, HIGH);

   rpmcount = 0;
   rpm = 0;
   timeold = 0;
 }

 void loop() {
   delay(1000);
   detachInterrupt(0);
   rpm = 30*1000/(millis() - timeold)*rpmcount;
   timeold = millis();
   rpmcount = 0;



   attachInterrupt(0, rpm_fun, FALLING);
 }