Wider does not always=better. You get a wider footprint with the 275 but the sidwall will flex more and the ride will be sloppier than something like a 245 or 255. IMHO if you are running all 4 rims the same size to rotate tires, run a 255/40/18 on 18s or a 255/45/17 if on 17s. I prefer staggered in which case I'd still do the 255 up front (or 245 if it was a better $$ deal) and a 285-315 out back.
2000 Mustang GT Steeda #0048 34k miles. 99 Cobra 4V swapped, FTBR IRS swapped, exhaust, tuned by James Gordon.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.