If cars were constantly 3d-printing themselves from the inside to replace every single part with a new one every day (as living multi-cellular organisms do), I'm pretty for them consuming other running cars as a source materials would be best (as cars would contain the necessary rubber and steel in proportions that are perfect for cars). You don't even need to consume fuel separately — your car would re-cycle fuel from the consumed car's tank — sorry for continuing your metaphor.