Wheel Encoder Tick counter error

I am trying to build a robot car to move forward a prescribed distance using 20 slot encoder disks and a IR interrupters. I am getting much higher values of ticks than I am supposed to for a rotation. Is my code correct?

///VARIABLE SETUP FOR ARDUINO I/O AND SENSORS

////////MOTION CONTROL VARIABLES///////

//Motor 1 which is the Right MOTOR

int enA=9;
int in1=8;
int in2=7;

//Motor 2 which is the Left MOTOR

int enB=4;
int in3=6;
int in4=5;

////////ENCODER VARIABLES////////
//Constants for Interrupt Pins (Used for the Wheel Encoders)

const byte MOTOR1=21; //Motor 1 Interrupt Pin
const byte MOTOR2=20; //Motor 2 Interrupt Pin

//Float for number of state changes in a revolution for the encoder disk

float enc_calib=40; //20 slots per disk so hence 40 state changes

//Constant for Wheel Diameter

const float wheeldiameter=66.10; // In mm

//Integers to pulse counter (Used for the Wheel Encoders)

volatile int counter1=0;
volatile int counter2=0;

//////INTERRUPT SERVICE ROUTINE FOR ENCODERS//////

//Motor 1 pulse count
void ISR_count1(){

counter1++;
Serial.println("ISR_COUNTER1=");
Serial.println(counter1);

}

//Motor 2 pulse count

void ISR_count2(){
counter2++;
Serial.println("ISR_COUNTER2=");
Serial.println(counter2);

}

// Function to convert cm to Steps

int CMtoSteps(float cm) {

int stepcount;
float circumference= (wheeldiameter*3.14)/10; //Calculate Circumference in Cm
float cm_step=circumference/enc_calib; // Cm per step

float f_stepcount= (cm/cm_step); //Convert cm/step to cm as a floating point
stepcount= (int) f_stepcount; //Convert float to integer....not rounded off.
return stepcount;

}

//FUNCTION TO MOVE FORWARD

void MoveForward(int steps,int mspeed){

counter1=0; //Reset Counter 1
counter2=0; //Reset Counter 2

//Set Motor 1 to move forward
digitalWrite(in1, LOW);
digitalWrite(in2, HIGH);

//Set Motor 2 to move forward
digitalWrite(in3, LOW);
digitalWrite(in4, HIGH);

while (steps> counter1 && steps> counter2){

if (steps>counter1) {
  analogWrite(enA,mspeed);
}else{
  analogWrite(enA,0);
}

if (steps>counter2) {

  analogWrite(enB,mspeed);

}else{
  analogWrite(enB,0);
}

}
//Stop Motors when distance reached

analogWrite(enA,0);
analogWrite(enB,0);

counter1=0;
counter2=0;
}

void setup()

{
Serial.begin(115200);

//Set Motor control pins to output

pinMode(enA,OUTPUT);
pinMode(in1,OUTPUT);
pinMode(in2,OUTPUT);
pinMode(in3,OUTPUT);
pinMode(in4,OUTPUT);
pinMode(enB,OUTPUT);

//Encoder Timer

attachInterrupt(digitalPinToInterrupt (MOTOR1), ISR_count1,CHANGE); //Increase counter 1 when speed sensor pin changes state
attachInterrupt(digitalPinToInterrupt (MOTOR2), ISR_count2,CHANGE); //Increase counter 2 when speed sensor pin changes state

// ///MOTION COMMAND//////
//
MoveForward(CMtoSteps(20.77),200);

}

void loop()

{

}

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If each wheel is 20 slots, why are you adding both wheels and counting to 40? It should be looking for 20 pulses for each rotation, for EACH wheel!

Additionally,
attachInterrupt(digitalPinToInterrupt (MOTOR2), ISR_count2,CHANGE);

Use FALLING, instead of CHANGE

Double check that logic my friend!

It is not advisable to Serial.print inside an ISR.

Note the fact that counter1 changed and use that to trigger a print.