Is SD central? Hard to say. Anyhow I have a nice 130 mile track out of Custer, SD, with all but a fraction on Dirt. There's gas at the Deerfield Lake Resort, that pops up about 40 miles from Custer, then again on the way back. Lots of graded dirt, numbered forest service roads with a smattering of medium technical stuff thrown in. This might be a good point to note that the BHNF, unlike most of its counterparts, has map files to load into your Garmin GPS that show the actual OHV trails. I hope that practice catches on among the other NF's, particularly the Apalachicola. For the BHNF, here's the page. Any questions, feel free to PM me.