I am a fan of
www.Fuelly.com. It's pretty easy to set up on a smart phone and takes about the same amount of time as writing everything down. Plus you can browse through the vehicles and see what others are getting on the same vehicle or others. I think that it can do more, but I just use it to track mileage.
My '96 5sp w/ 210K just showed 25.7 for 150 miles with a 50% mix of country highway and city driving.