Hmmm, *goes back and reads page 13 then page 12 for a light vehicle* Dang! I'm glad I haven't designed a towing ground vehicle yet. Otherwise, I'd be doing it wrong. Ignore what I said above.
Important thing to note before going on: a 1 space vehicle is a moped (off road being a dirt bike). Not exactly a "tractor" that is pulling farming equipment.
I'd say that more likely, there are a lack of Universal Options that are appropriate for a farming vehicle. Some things I would say are appropriate to use some of those spaces for a tractor:
- Easy to repair (see high guard for basic idea)
- Durable (see high guard for basic idea)
- Greater fuel capacity
- lower fuel efficiency
- increased stability (aka big tires, basically a double off road)