I've been researching tire options for 2018 Q-7. I'm particularly interested in All-weather (NOT all-season). It seems that there are no options at OEM size (285/45-20), but many options if I change to 275/45-20. Obviously, this would make speedometer read over actual speed. When I discussed this with a few service advisors, one said this would interfere with anti-lock brake system and void warranty.

This makes no sense to me. How can ABS detect overall wheel diameter?