Tachometer results not stable

Projects General Guidance
3

The fluctuations come from a big part of the math, the faster you want to measure the more inprecise the measurements will be

if you display per second and you have a measurement of R pulses in one second you multiply by 60, if there is one pulse more or less that means on the RPM scale you get + or - 60 !! that is quite a difference

Here a minimized version of your code that shows this effect

#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 rpm_count = 0;
unsigned long prev_rpm_count = 0;

unsigned long disp_timeold = 0UL;

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


//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;
    
    unsigned long t = rpm_count;
    Rpm = t - prev_rpm_count;      // rounds made since last display
    prev_rpm_count = t;            // remember previous counter
    
    Rpm = Rpm * 60000UL / timetemp; // convert to rounds per minute

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

//FUNCTIONS________________________________________

void rpm_interrupt() 
{
  rpm_count++;
}

another version is coming soon