I'll teach you a way: fill it up tomorrow (jump gun) and write down the current mileage A.

Run for a while, it's best to run until there is little oil left in the tank, and then go to the gas station to fill it up (jump gun) to see how many liters of oil the gas station shows, assuming it is m. Then write down the current mileage B.

Fuel consumption per 100 kilometers 100M/(B-A). This method is very accurate.