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It appears that the two vendors you're looking at have different ideas about fitting tires to wheels. Appearance is likely part of the reason behind recommending the 285-wide tire, while "crisper" cornering behavior is apparently something that the guys suggesting 255's are more concerned with.
285/40-20 is generally accepted for fitment to rims between 9.5" wide and 11" wide.
255/45-20 is generally accepted for fitment to rims from 8" wide to 9.5" wide. It appears that this vendor is willing to stretch them another half inch. The first half inch "over-max" isn't something to get very excited about, as the "go/no-go" line for mass market tire size decisions has to be drawn somewhere.
In most cases, mounting tires on wheels half an inch above the widest recommended width is better than mounting the same tires on wheels that are half an inch below the minimum recommended width. At least from a sidewall heat buildup point of view, which is arguably more critical than either ride quality (firmer) or rim scuff protection (not as good).
Actual tire measurements are permitted to vary slightly (there's a spec for this permissible variation). Tires of a given size from some mfrs run wider than do the same size tires from other mfrs.
Norm
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