[clyde]

I keep logs in all my cars that include overall miles, miles on that tank, date, brand/location of gas station, how many gallons, and $/gal. In cars with trip computers, I also include the OBC MPG and average MPH for that tank. I don't note anything else (driving conditions, weather, etc). In a separate log, I track maintenance and repairs. Every so often, I merge it all together.

What to track depends on what you want to do with the info.