It's about 60 miles east of Fresno in the lower Sierra Nevadas. Also, just south of Yosemite as the crow flies. Here's a link to some grid coordinates and a map.
http://www.tnt-jeeping.com/mirror.html
There's so many trails up there you could spend a month trying to wheel all of them. This is probably one of the tougher ones, but then again I've only been on 4 so far. The longest one I've done was the Swamp Lake trail and it's 13 miles. Next year I'd like to try the Dusy/Ershim route. It's 31 miles, but I'm going to have to get lockers and some more armor. Everyone I've talked to says it's tougher then the Rubicon.