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Well you have 17" rims and bought 18" rims so the stock sized tires will not fit.

To keep the circumference of the tire similar to stock and keep your speedometer as accurate as possible you will need 215/40/18. With that your speedometer will be 0.6% too slow. When your speedometer reads 60mph you will actually be traveling 60.4mph. That's the closest you will ever get.

If you want a wider tire for more grip you can step up to a 225/40/18 and that will be 1.9% too slow. When speedometer reads 60mph you will be traveling 61.1mph.
 

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really? speed depends on width? i always though it was the height/profile (40-45).

Yes because the sidewall aspect ratio changes with width. It's the percentage of the width not a straight measurement.

So a 225/45 tire is taller than a 215/45 tire because that's saying the sidewalls are 45% of the width. So 45% of 225 is more than 45% of 215.
 
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