fuel consumption 30L

Hello, I own a 2005 model 407 2.0 HDi Executive. The trip computer shows that it consumes approximately 10-12L of fuel on normal roads at low speed, 20-24L at low speed, and 30L when going uphill. This car is planting a fig tree in my garden. I drive an average of 10km a day in the city (for work and back), but I pay 600₺ for fuel every 5 days. What could be wrong with this car? I wanted to diagnose the problem first because I don't want to go to the Sultanbeyli industrial site and get scammed. Does anyone have any information?

The car has a four-speed automatic transmission, and my normal fuel consumption is 22-25 mph. After an oil and filter change, it returned to these levels. Is there a reliable mechanic on or around Istanbul's Anatolian side? No matter where I take it, I'm constantly fed up with attitudes like, "This is a 2000 engine, we can't service it, it's a 407, it's a problem, I don't understand it."