It is an old engine, most likely driven for over 200k km (dont worry about what the odo says). Even new, these engines were more renowned for performance, and less for fuel efficiency. There's nothing much you could do really. Better enjoy it during your trip to the North, as this engine and your vehicle are best suited for the terrain.
I used a surf with a/t and 1kzte, with mileage 195k, and it never went above 6km/l, whether driven carefully or fast, empty or with full cargo.
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