I fill the tank slowly until the pump stops. The trip meter is reset and off I go. I might fill-up again when the gauge gets to 1/2 full or close to empty. Either way, I refill using the same technique, then divide that amt. of fuel(gallons in my case) into the miles traveled.
You mention litres; I suspect you're outside of the U. S.
If your car's trip meter accumulates kilometers vs. miles,
you could use a similar strategy, but divide kilometers
traveled by litres added at fill-up.
Fill-up and drive your usual 70-80mph/kpg/whatever. At the next fill-up, calculate your litres per kilometer. Now drive for a tankful at a reduced speed. Fill-up and recalculate.
1991 300-SEL - Model 126
M103 - SOHC
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