how wide is your rim? most aftermarket rims are 7.5" wide (17 X 17.5)
if so, then i would get 225/40/17
if they are 6" i would get 205/40/17
if they are 6.5" i would get 215/40/17
and if they are 8" i would get 235/40/17
you can always hedge a bit either way...for instance on a 7.5" wide tire the usual available options are from 205/40/17 to 235 245/40/17 ...and you can eve hedge on this a bit