I need new rear tires for my Coupe. I have put on about 28,000 miles. The fronts are fine.

Since the fronts are Michelin, I guess I should have Michelins at the rear too, right? I got Pilot Sport A/S 245/45WR18 from the factory. Do I have to replace them with the same or can I substitute Pilot Sport A/S 235/50WR17 for them?

What is a good place to buy them at a discount?

There is a $100 price difference between Michelin and Avon tires. Do I have to have Michelins or can I get all 4 Avons? Although Avon is not a known brand name, they are rated the same or higher then the Michelins (I don't want to pay for a name). Also, the Avons are all-season, whereas the Michelins are summer-only. The UTQG treadwear rating on the Avons is 360 whereas on the Michelins it is 220.

Thanks.