I happen to be on the far end of that spectrum.
I have kept track of every penny I have ever spent on my truck for 100% of every category since the day it was new, August 15, 2006.
I can tell you exactly how many liters of diesel I have put in it and how many times I've put fuel in it. I have a log book for it all.
I have tracked my MPG since new and can tell you that it has actually gone up in recent years rather than down.
For me, it is my work truck and 100% writeoff at tax time.
So, why do I do it? Because I can, and I enjoy it, I like statistical facts about my own vehicle. I like to know how much life I get out of a set of wiper blades, or a pair of batteries etc etc. I enjoy it. So why not?. I have all the receipts for everything spent too.
One example: My Michelin tires have exactlty 129,571K, or 80,511.68 miles on them. Not bad for an always loaded dually work truck.
Some of you are shaking your head right now, I just know it! :B
I also found a wonderful, amazing app for Android that helps me keep track of things. I like it so much that I took my paper logs and went back to day 1 and entered everything into the app.
Close studying of this data has taught me the sweet spot of driving speed for my particular rig, where I get the best fuel mileage.
I do it because I'm a geek and I get a kick out of doing stuff like that.:)