I am only using them in certain circumstances. They will be better on snow/dirt/mud than my stock Bridgestone Dueler A/T tires on 17" wheels. The setup above should give me an inch in diameter increase and 20% more width if I remember correctly. From what I have seen on vids/pics there should be no rubbing on turns.If you're getting the BFG KO2's, they're not rated off-road/snow only. You can use them year round on and off road. Or are you only using them for that purpose?
Either way, yes your calculation is correct. The difference in the speedometer will be minimal. Not enough that I'd re-calibrate for it either, even if I wasn't using them for off-road/snow only.