BFG Stock tires run small?

I just measured my 315 bfg at. and mounted on the truck, they measure barely over 33" tall. Is this normal?

You need to put more air in them - that'll make them bigger. Pressure them up until they measure out at 35".

Ok, really, yes, the 315 BFG AT tend to run small. But you're also measuring "tall" which tells me you're measuring height. This will be under since part of the height is compressed by the truck's weight flattening the bottom radius of curvature of the tire. Try measuring the diameter across the tire, not vertically. Oh, and don't inflate until 35" diameter is achieved - that's just bad advice.