It's according to what your doing with the trucks. I have a farm tag on my small dump truck and it save me a good amount. When I was checking out the legalities and requirements the DMV guy I spoke with was very helpful until I told him it would be for farm use only. He then told me I'd better not ever get caught hauling gravel for hire and hung up the phone.
I believe you still have to sign an affidavit at the tag office stating that you farm a minimum amount of land. I don't remember the amount though.