Personally I'd just put it into a high yield savings account where you can make "sub-accounts" like Capital One 360, you can call it "Car hobby" and that money will be marked as a sub-account of your savings. You can budget $X amount for it each month by using YNAB (just google it).
If you want to put it into something longer term like an individual investment account, I don't know if Vanguard or Fidelity allow you to create sub-accounts. Fidelity doesn't, at least. If you're absolutely set on doing it this way though, just budget for your contribution for it using whatever budget software you use, and look back at your contribution statements from the previous years and add it up.