if you have a 4x4 (4 bolts on 4" centers) hubs now, they make an 8" trailer tire that has road worthy tread and a little grip, should be ok as long as it isn't horrifically muddy.
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=44143
is an example, since the tire is road rated, they will probably last longer assuming you don't smoke them alot. its an 8" rim, tire is about 13" tall more or less, so if you don't have 4x4 bolt pattern, get an 8" rim to fit your hub and you can get them at Tractor Supply or almost anywhere tires are sold (may have to order them if not getting it with a rim though, but they are stupid inexpensive, like $20 or so)
depending on the size you need, look into lawn tractor tires, come in various treads and usually can be had pretty inexpensively, also GOLF CART TIRES if you want something wider than the trailer tires above. since you said "dirt road" you need enough tread to get up the hill, but its a "road" so focusing on the the on-road riding will get you better tire life, trailer tires and golf cart tires are somewhat designed for the paved and "tame" off-roading surfaces, so they will have longer life than most anything else...if you want to get a set of tires/wheels for when you trailer somewhere to go off-roading, that would make more sense than ruining more expensive tires more often. IMO
