Important thing is to check petrol u are using...
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For e.g. I get good petrol last weekend, travelled on motorway, of gave me average of 18.5 from Islamabad to Lahore with 5 adults on board
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On way back, petrol quality wasn't good, it gave me 14.5 with 2 adults and 3 children and some luggage
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Again got good petrol yesterday, got average above 22 on GT road with one person onboard...