Although your method of calculating the total petrol quantity your mehru used to cover the distance, it is still very nice fuel avg for a carby mehru. I myself used mehru for a few months and never came close to 20km/liter. Even with minor correction of petrol, its a very good avg you got. But also remember, you won't get the same mileage beyond 75km/hr avg speed. Trust me carby mehru drinks petrol like anything once driven above that speed.