My impreza 1.6TS comes with 185/70 R14" i am planning to have bigger wheels but i am not sure about the tyre size i am planning to put 17" in the stock WRX they are putting 215/45 while if i did the calculations of the diamater i should be putting 205/45 so which one should i use inorder not to screw the speedometer or screw any thing

 

The difference is negligible, an error of +1.0mph with the 215 compared to +0.2mph with the 205. Odds are your speedo is off by that much to begin with, go with whichever size you like more. Tire rack has a wider selection in the 215 (66 vs 21), so that may also be a factor in making your decision.

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