ya know just take a look at a gear chart. Remember those ratios are considering a 1:1 at trans, in 5th its .79 or.84...? anyhow just look for your stock size look at rpm go down to new tire size and look at rpm. go over till you get close to stock rpm.
my dad went from 205 to 235 and cant run 5th gear, motor lugs to much, but he has a 4 popper and cant get the torque to maintain such a low rpm.