[bouncytruck]

I've had Blazers with 35's on 4" and 6" lifts. I'm guessing you'll need at least 10" for 40's if you plan on taking it off road. There was a guy around here who had 40" tires on a 6" lift. The tires almost hit the body. He said that the suspension was so stiff that the flex of sidewalls on the tires was the only flex. Sounds like miserable ride.

Keep in mind with a big lift and big tires you may need to upgrade your axles, re-gear, add cross-over steering and add more braking power.