I noticed that you don't have the option to tag something "Done" in the project planning screen. You can be planning it, doing it, or testing it but you can't tag it done.

Looks nice. I use an excel sheet for mine right now.

For fuel consumption I track a running average of the last X fillups where X is equivalent to the oil change interval. That helps identify performance issues as you can see your fuel economy decreasing as it nears time to replace plugs, air filters, etc. I also break out parts replacement by system - transmission, engine, transfer case, differentials, wire harness, lighting, etc just as you would use in ordering parts. That way specific items are easier to find in the maintenance records just by searching records for specific systems. It helps to understand the expected lifetime of your parts and to spot things like differences between OEM and aftermarket parts from specific parts suppliers.

Good job with this.