Ok, 61c/Km
€6.10 / 10Km
€6.10 = approx 5 L of Petrol (@ €1.22/L)
5L will easily get an average modern (or even 10 year old) car 40-50Km so I think you're doing pretty well as it is, mileage-wise, without looking for more.
If you do 500Kms, you'll get €305.
Assuming 50Km/Gallon (a conservative estimate as that's poor enough mileage nowadays) you'll need 10 Gallons, or 45.5 litres.
Again, assuming 1.22/l, you will pay €55.51
This leaves you with a profit of €250 for 500kms.
€500 would be the price for an average annual service including service parts.
So 1000kms for your employer will pay for an annual service due every 12-15000 kms.
Not a bad deal, I'm sure you agree and one that should satisfy you without looking for any more on top of that for 'wear and tear'.