# 3. Mileage CalculationΒΆ

With the ability to calculate speed, counting miles is really a piece of cake.

In this case, the mileage is calculated by multiplying the total number of revolutions by the circumference of the wheel.

```from machine import Timer, Pin
import math

WP = 2 * math.pi * 3.3 # wheel_perimeter(cm): 2 * pi * r
duration = 200

def on_left(ch):
global left_count
left_count += 1

def on_right(ch):
global right_count
right_count += 1

def on_timer(ch):
global left_count,right_count, total_count
# mileage
total_count += left_count + right_count
mileage = total_count /20.0/2 *WP
# revolutions per second
rps = (left_count + right_count) * 1000 /duration /20.0 /2
# speed
speed = rps * WP
# clear count
left_count = 0
right_count = 0
print("mileage(cm): ",mileage," ; speed(cm/s): ",speed)

# Interrupter, used to count
left_count = 0
right_count = 0
total_count = 0
left_pin = Pin(8, Pin.IN, Pin.PULL_UP)
left_pin.irq(trigger=Pin.IRQ_FALLING, handler=on_left)
right_pin = Pin(9, Pin.IN, Pin.PULL_UP)
right_pin.irq(trigger=Pin.IRQ_FALLING, handler=on_right)

# Timer, print speed
tim = Timer()
tim.init(period=duration, mode=Timer.PERIODIC, callback=on_timer)

# motor run
import motors
import time

try:
while True:
for i in range(20,100,10):
motors.move("forward",i)
time.sleep(1)
finally:
motors.stop()
time.sleep(0.2)
```

Copy the above code into Thonny or open the `speed_3_get_mileage.py` under the path of `pico_4wd_car-v2.0\examples\learn_modules`. Then click the button or press `F5` to run it.

After powering up the Pico 4WD car, you can see that the speed keeps cycling between high and low, and Mileage keeps adding up unless you click to stop the script from running.