If you want to fiddle with estimating your tire size, RPM, and speed, try this calculator. I like it a lot. If your transmission isn't one of those on the selection list you can put in your own gear ratios (and rear end ratios too) manually, so it will work with any combination you like.
The calculator says 26.5" tire, 3.73 gears, 4000 RPM gives you 85 MPH. I think
@33IMP is right, the math doesn't pencil out - you likely have a bunch more gear than 3.73's. You might want to get someone to pace you at whatever speed, then check your RPM. From that we can figure out a decent guess what your gears are.
Speed/Tire/RPM calculator