Just to make it official... /forums/images/graemlins/laugh.gif
From right triangle math: c = b/COS A. c = width of blade. A = blade angle, set to 30 degrees. b = rear tire outside to outside width.
c = b/.866, or c = b * 1.155.
For a 5', or 60" tread width, c = 60" * 1.55 = 69.3". Rounding up to a 6', or 72" blade width, that leaves about 1.4" of extra coverage outside each wheel.
Jumping up to a 7', or 84" blade width, you end up with about 7.4" extra coverage outside the tires. That gives a bit more tractor clearance to blade things away from fences or buildings.
'Course, if your blade will easily offset, stepping up to a 7' blade for a 5' wide tractor might not be absolutely essential.
Still, if you were to use the blade reversed to push snow in reverse rather than pulling it through the snow moving forward, you'd definitely want to make sure it covers your tread width so your tires are not having to dig through fresh snow. This will let you use the rear blade in reverse to move snow that might be to deep to drive through going forward.
