The only way to figure it out is to test. Fill it up, drive 100km, fill it up and figure it out. I bet the number will be about 26 american miles per american gallon.

To increase effeciency you could
- switch to synthetic oil
- remove the EGR, but I doubt that you have one
- make certain that your air and fuel filters are clean
- get good tires (Michelin Energy)
- drive on smooth roads with the wind at your back and behind big trucks.
- possibly drop your cars height, but really not worth it
- drive around 2500 rpm in top gear
- you get the idea

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84 300TD - 235k - Farbe "Surfblau" bought at 213k
87 300SDL - 131k - Farbe "PimpGold" bought at 115k
00 Klepper Faltboot Expedition Double