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i have 14s right now on my civic ex. can i possibly fit 13s? i plan to get 225/45/13 inch tires on there for better handling and autox like performance

 
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No. The brakes are too big. And the only tires I'm aware of that are 225/45/13 are race tires.. I seriously hope you have no intention of driving those on the street. lease:

 
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its for autox if i'm not mistaken. if they allow it

 
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You're allowed non stock sized tires in all but the stock classes. You're not allowed anything wider than 225 in ST classes, but it needs to be have a treadwear above 140, which no race tire does, which leaves the SP classes, where you are allowed to use R compounds, but its no place for a nearly stock car. Most cars that fair well in SP are to the point of trailer queens, with tons of motor and suspension work done. Farther than that is SM, but theres no place for a non turbo or swapped car up there unless you like getting your *** handed to you.
So stick with 14's or 15s and a set of Azenis or Kumho MX and run STS. Consult the link in the Autocross FAQ page in the suspension forum for the full set of rules. But you have 0 chance of fitting a 10.3" rotor + its caliper underneath a 13" rim.

 
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from what i have seen, if you have a rotor of lets say 10 inches, the rim you have to get has to be atleast 3.5 inches bigger to clear the caliper

so if you have 10.3 you need 10.3+3.5= 13.8 so 14's would be the lowest.

for some wilwood big brakes those are 12.3 rotor so 12.3+3.5= 15.8 so you would barly fit those under 16's if you get the right offset

 
Rep Power: 380

14s are the smallest wheels you'll be able to fit.... the only Issue I see with runnig 14s is finding a wide 14 wheel. You'd wan't at least a 14X6.5, but a 14X7 or 14X7.5 would be much better, I'm not sure we can fit an 8' wheel. In the wider wheels you'r pretty much looking at custom race wheels from a company like Keizer or Kodiak. and they arnt cheep Their 15X7.5 run about $350 each.

For us a light 15 is the way to go. that way you can find a cheep 15X7.5 and wrap it with a wide short tire..... cheep and easy. That is unless you can afford a set of Kodiaks....