I'm guilty of recording. And being a computer guy I recorded on a spreadsheet that automatically calculated my MPG and such. I would fill up and write the trip meter mileage down on the receipt and then reset the meter. At home I opened the spreadsheet about once a month and entered the date, miles driven, gallons filled and the cost per gallon and if on a trip a note of where I filled. The spreadsheet did the calculations after that.
I remember as a teenager I had a ford ranger that I would track this way. One day the gas gauge just quit. I knew that I could drive 169 miles until empty. Knowing that and being a teenager I drove that truck for years with no working gauge and never ran out of fuel.
Here's a small snippet of my spreadsheet.