whichever is closest to stock tire/rim combo size....which i believe would be the 40's.....i am almost positives....the 45's on my car make my 16's really close to stock size. So i imagine 40's would do the same on 17's.
For the most accurate mph, 225/45/17 is closest to stock. Next is 215/45/17. If you want to see exact details, go to http://www.whalentire.com and look at tires. It will tell you the size choices recommended for 17" rims and the difference in mph based on the size you choose.
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2004 WRX - WR Blue Pearl 1997 Eclipse GS - Royal Sapphire Pearl