If it`s just mileage why not use a simple Excel file. No bells and whistles but works.
I created one that checked everything. MPG, Litres, cost per gallon/litre, etc with full expenses logs and end of year summaries with actual running costs per mile not just petrol cost per mile. But then it is my car and I charged the company. Not particularly user friendly but once you know your way round it it worked for me.