What do i need??

Right now i have the 10 spoke SS rims on my car with 275/40/17 on all four rims. I need new back tires and i want to go a little wider like a 285. Do i put a 285/40/17 or do i put a 285/35/17 on the back to match the front?

First question would be "can you buy 285/35-17's?" Tire Rack doesnt' show any tires in that size.

Then.... a 285/35-17 would not "match the front". It would have a much thinner sidewall, and would be more than 3/4" smaller in diameter than your front 275/40-17. That would increase the gap to the wheel well by almost 1/2".

The 285/40-17 is the way to go, but once you pick a tire brand, look carefully at the dimensions. While all of the 285's are going to be slightly larger in diameter than your 275/40's, not all 285's are exactly the same width, so you might not actually gain anything compared to tire you have on the front now. You would also be giving up the ability to rotate the tires, and its really a good idea.

Don't buy a cheap brand of tire and you'll usually get a wider tire. For the minimal difference of a 285 vs 275 you should check the differences in tread width between brands first.

thanks, i'll keep all that in mind.