Tachometer results not stable

Projects General Guidance
4

This variation does measure the time it takes to do one rotation and measures the RPM from the very last rotation. It is definitely a different approach

#include <Wire.h> 
#include <LiquidCrystal_I2C.h>

LiquidCrystal_I2C lcd(0x27,16,2); //set the LCD address to 0x27 for a 16 chars and 2 line display

//TESTPIN
const int testpin = 3;

//MAIN INPUTS
const int DispSampleTime = 1000;               //[millisec] //= 0.1sec 

// variables:
volatile unsigned long lastTime;
volatile unsigned long thisTime;
volatile unsigned long duration;

unsigned long disp_timeold = 0UL;
unsigned long timetemp = 0L;   


//SETUP RUN ONLY ONCE @ start
void setup() 
{
  //DEBUG
  TCCR2B = TCCR2B & 0b11111000 | 0x07 ;  // sets PWM to 30.517578125 Hz on pin 3 and 11 //timer 2

  //testpin TESTESTESTEST----------------------<<<<<<<<<<<<>>>>>>>>>>>>>
  pinMode(testpin, OUTPUT);
  analogWrite(testpin, 1);    //sample 30.51757 hz pulse 1/255 uptime.

  lcd.init(); // initialize the lcd
  lcd.backlight();
  
  //set interrupt on pin 2 and call function rpm_interrupt when pin is falling
  attachInterrupt(0, rpm_interrupt, FALLING);
} 

void loop() 
{
  // DISPLAY RPM
  timetemp = millis() - disp_timeold;
  if( timetemp >= DispSampleTime) 
  {   
    disp_timeold += timetemp;
    
    float RPM = 60e6/duration;  // 60.000.000 micros / duration in micros

    //disp stuff
    lcd.clear();
    lcd.print("Rpm:  ");
    lcd.print(RPM);
  }
}

void rpm_interrupt() 
{
  lastTime = thisTime;
  thisTime = micros();
  duration = thisTime-lastTime;
}