19.5" is what I believe the UPS trucks are, or were, using. 19.5", 8 lug dually wheels were used on Chevrolet and GMC one tons in the 50s (late) and 60s (seen 'em on '57 - '69 model years, though not sure which years were factory equipped).
So, other 19.5" wheels are out there. And high-profile tire selection may be greater than with 18".
