in thinking of simple math (round numbers), say the tire diameter is 25", it is easy to calculate the nominal tire profile above the rim diameter
with 14" wheels, the tire profile is 5.5"
with 15" wheels, the tire profile is 5"
with 16" wheels, the tire profile is 4.5"
with 17" wheels, the tire profile is 4"
with 18" wheels, the tire profile is 3.5"
so generally 14" wheels for the e9 use 70 series tires / 15" wheels use 60 series tires / 16" wheels use 55/50 series tires / 17" wheels use 50/45/40 series tires and 18" probably use 45/40/35 series tires dependent on the width size available.
i thought about 17" with 50 / 45 series ... but 1.5' Sidewall profile difference ... for me 50 series tires are comfortable. 60 series are more comfortable and 70 series tend to slide a bit on really twisty mountain roads -
@Stevehose, talk to us about Tail of the Dragon ... of course you were using the CN-36's which are much better than your basic grand touring all season tires.