does a vehicle have to be a certain weight to write it off?

Answer 1

Gross weight >6,000 and the limit is $25,000 unless it is a special use vehicle, i.e. hearse, taxi, ambulance.

Answer 2

Technically, you can't write it off, you depreciate it. Technicalities aside, yes, it must weigh at least 6,000 lbs. Be aware, however, that the rules on that particular issue change often (lately, almost every year).