Depends how generous we are with the definition of door. Do we consider any sort of opening a door? Is the way bugs get into the ground a door? If so, then doors. Otherwise it's probably wheels. I think the general definition for what we consider a wheel (without getting really loosey goosey like I was before) is much more broad. A door is something for a person to open and close. A wheel is a round thing that spins. Even if you're thinking something like "well, buildings have a lot of doors, and there are more buildings than cars..." Consider that a lot of furniture has wheels. Drawers have wheels. My dishwasher has a "door" but the racks have wheels. My office chair has 5, maybe 10 depending on how you count. I've got like four in the house I think. That's already like 40 wheels from just office chairs.