I used wantai Stepper motor(.9/step) in speedometer.but?

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int rpmcount = 0; 
int dirPin = 3;
int stepperPin = 4;
int a=0,b=0,c=0;
int rpm = 0; 
 
 
unsigned long lastmillis = 0; 
 
 
void setup(){ 
 Serial.begin(9600); 
 attachInterrupt(0, rpm_fan, FALLING);
  pinMode(dirPin, OUTPUT);
  pinMode(stepperPin, OUTPUT);
  Serial.begin(9600);
  
 } 
 void step(boolean dir,int steps){
  digitalWrite(dirPin,dir);
  delay(50);
  for(int i=0;i<steps;i++){
    digitalWrite(stepperPin, HIGH);
    delayMicroseconds(100);
    digitalWrite(stepperPin, LOW);
    delayMicroseconds(100);
  }
  
}
 
 
void loop(){ 
 
 if (millis() - lastmillis == 1000){ //Uptade every one second, this will be equal to reading frecuency (Hz).
 //step(true,2000);
 // delay(10);
 // step(false,2000);
 
 detachInterrupt(0);//Disable interrupt when calculating
 
 rpm = rpmcount * 60; // Convert frecuency to RPM, note: this works for one interruption per full rotation. For two interrups per full rotation use rpmcount * 30.
 
 Serial.print("RPM =\t"); //print the word "RPM" and tab.
 Serial.print(rpm); // print the rpm value.
 Serial.print("\t Hz=\t"); //print the word "Hz".
 Serial.println(rpmcount); //print revolutions per second or Hz. And print new line or enter.
 a=rpmcount;
 c=b-a;
 c=c*100;
 
 if(a!=b && b>a)
 {
   step(false,c);
  
 }
 if(a!=b && a>b)
 {
   step(true,c);
   
 }
 if(a==b && a==0 && b==0)
 {
 
 }
 if(a==b && a!=0 && b!=0)
 {
 
 }
 
b==a;
 
 rpmcount = 0; // Restart the RPM counter
 lastmillis = millis(); // Uptade lasmillis
 attachInterrupt(0, rpm_fan, FALLING); //enable interrupt
  }
 
 } 
 
 // this code will be executed every time the interrupt 0 (pin2) gets low.
 
 
void rpm_fan(){ 
  rpmcount++;
 
 }