So I recently bought 2004 Aveo Manual 1.6L in Canada.

The car gave me 350km in a full tank (40L) on highway.

Then It gave me 280km in city with a full tank.
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I decided it needs maintenance, although the oil was relatively new, i still decided to change it.

I put Castrol Advanced Synthetic Oil in it. With a filter from FRAM and an air Filter from FRAM as well.

Used an Engine Flush liquid to clean up the engine

and put NGK platinum spark plugs in it.
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I filled up the tank with High octane 91 Gas, and It gave me 400km in a full tank in city,
I am still not satisfied with the performance as I drive a manual car a civic/corrolla/mazda3/acura gives better mileage than this car as I discussed with my friends who own these cars.

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Does someone have suggestions for me, i expect atleast 500km in a full tank, does anyone get than mileage? If you get it then how?