See? I knew you did care, just needed a minute. LolYour shop obviously cares very little about your safety...mounting a min. 8.5 on a 7.5 rim is risky.
Load rating should have almost nothing to do w the matter
And here's a like for you in return.
I'd be very interested in getting a response from discount tire on the matter. They have 7.5" width as the minimum required for C rated 315's on their product page.
Have a great day, Noah!