Just has to do with them being totally different cars with different OEM tires, etc. You may be able to squeeze wider than a 285 on an 05+. I have no idea, but as for the 94-04 cars a 315/35/17 is the same overall diameter as the stock 245/45/17.
You have to make them as close as you can or your mileage and speedo will be off. Maybe the 285/40 tire works out to the same as the stock size on the 05+
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