I use a programme downloaded from CarCare.com. Works out everything for you in any combination of units (litres/KM, Gallons/Miles and Litres/Miles) brilliant. Good personal support from the writer who tweaked some bits for me with a patch.
Are you sure that's the correct site? If I go to www.carcare.com I get redirected to a site called firstplace.com which looks very much like one of those spam web sites set up to fool search engines.
A search of my own did reveal a beginner's guide to creating your own spreadsheet that would serve the purpose: tinyurl.com/2c2cs (Acrobat file). I'm not a spreadsheet expert but this doesn't look terribly hard.
I've been trying to develop an Excel spreadsheet to do the car expenses for my husband's business. May I send what I did to you for some help? I'm getting lost at the more than 10000 miles calculation - I'm okay up to then. Look forward to hearing from you.