I take a picture of mileage, top up to full (sometimes to 2nd click, depends on the car), write down mileage and amount of fuel. Next time the same procedure and I have number of miles driven and amount of fuel used.
I use German website for storing fueling records, one in my signature. You can export them to Excel if you want, to keep data locally. I think, there's also website called Fuelly, but I never used it. In past, I had Excel sheet for that. I still use the same Excel sheet for writing down all costs (insurance, service, finance, cash purchase/disposal, etc). When I dipose of the car, I calculate cost per mile, per month, year, etc.
I use German website for storing fueling records, one in my signature. You can export them to Excel if you want, to keep data locally. I think, there's also website called Fuelly, but I never used it. In past, I had Excel sheet for that. I still use the same Excel sheet for writing down all costs (insurance, service, finance, cash purchase/disposal, etc). When I dipose of the car, I calculate cost per mile, per month, year, etc.