I replaced the radiator last year and the car continues to run cool.
When a car runs cool, it uses more fuel.
If your thermostat is original, replace it. In my experience, 60kmi is how long it typically lasts. It is inexpensive and easy to replace on most of the 4-6 cylinder gas models.
When the thermostat gets fatigued, it cannot close completely, so your car cannot reach or takes a long time to reach the operating temp. That makes your fuel mileage lower. Different gasoline could be a factor too but I doubt it will be a 20% reduction.