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Some Nissan models need the air-flow meter to be learnt when it's replaced but I am not aware of this for replacing the air filter as the air-flow meter will automatically have a different reading with more airflow going through the sensor.

I wouldn't suggest that you carry out any re-learning procedure as you don't have the diagnostic equipment to confirm whether you have correctly re-learnt the parameters.

But as I said in the beginning, your X-Trail's fuel economy sounds realistic.

All right then thanks a lot Donovan :)