You have to account everything, not just fuel, from car depreciation, servicing (oil changes,...), normal wear (brakes, tires,...), occasional replacements (batteries, wipers, ...), etc. If you drive 20.000kms per year, and a yearly service (oil, filters,...) costs 200eur, just that adds aditional 0.01eur/km. Two sets of tires (winter/summer) are ~2300eur + 8 changes (8~40eur) adds an additional (almost) ~1k eur in four years, 1 more cent. Insurance, registration.. 3, 4 cents. etc.
*that's the untaxed amount you get reimbursed for if you use your own car for a business trip.