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I have listed the most common WRX/STi tire sizes to show the overall diameter differences. This is just a basic guide to give you an idea. Obviously the rim width will have a great bearing in the diameter. Larger than stock diameter will change the final drive ratio to a lower number (taller gear), smaller diameter will change it to a higher number (shorter gear). Sorry if this is too basic but it may help some.

225-45X17 24.97" (Stock STi)
235-45X17 25.33"
245-45X17 25.68"

225-40X17 24.09"
235-40X17 24.40"
245-40X17 24.72"

225-40X18 25.09"
235-40X18 25.40"
245-40X18 25.72"

The formula used to caculate the above diameter is this: The rim width is 85% of the tires section width. Example: A 225-45X17 tire has a section width of 225mm or approximately 9". 85% of 9" = 7.65". Closest standard width rim would be 7.5". A 235mm width requires at least a 7.99" rim, a 245mm will require a 8.33" rim. For the above tire diameters the following wheel widths were used: 225mm = 7.5", 235mm = 8.0", and 245mm = 8.5".
 

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Sorry if this is too long. I found a website with a nice chart.
17 INCH
TIRES TIRE
HEIGHT
225 35 17 23.20
245 35 17 23.75
315 35 17 25.68
335 35 17 26.23

205 40 17 23.46
215 40 17 23.77
225 40 17 24.09
235 40 17 24.40
245 40 17 24.72
255 40 17 25.03
265 40 17 25.35
275 40 17 25.66
285 40 17 25.98

205 45 17 24.26
215 45 17 24.62
225 45 17 24.97
235 45 17 25.33
245 45 17 25.68
255 45 17 26.04
305 45 17 27.81

205 50 17 25.07
215 50 17 25.46
225 50 17 25.86
235 50 17 26.25
245 50 17 26.65
255 50 17 27.04
275 50 17 27.83

215 55 17 26.31
225 55 17 26.74
235 55 17 27.18
245 55 17 27.61
255 55 17 28.04
275 55 17 28.91

215 60 17 27.16
255 60 17 29.05
275 60 17 29.99
305 60 17 31.41

235 65 17 29.03
245 65 17 29.54
275 65 17 31.07

265 70 17 31.61

255 75 17 32.06

18 INCH
TIRES TIRE
HEIGHT
285 30 18 24.73
295 30 18 24.97
315 30 18 25.44
335 30 18 25.91

215 35 18 23.93
225 35 18 24.20
235 35 18 24.48
245 35 18 24.75
255 35 18 25.03
265 35 18 25.30
275 35 18 25.58
285 35 18 25.85
295 35 18 26.13
335 35 18 27.23
345 35 18 27.51

215 40 18 24.77
225 40 18 25.09
235 40 18 25.40
245 40 18 25.72
255 40 18 26.03
265 40 18 26.35
275 40 18 26.66
285 40 18 26.98

215 45 18 25.62
225 45 18 25.97
235 45 18 26.33
245 45 18 26.68
255 45 18 27.04
285 45 18 28.10
295 45 18 28.45

235 50 18 27.25
255 50 18 28.04
285 50 18 29.22

225 55 18 27.74
255 55 18 29.04
275 55 18 29.91
305 55 18 31.21

245 60 18 29.57
265 60 18 30.52
275 60 18 30.99
285 60 18 31.46
315 60 18 32.88
235 60 18 29.10
 

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Here's another pretty good calculator for tire diameters, circumferences etc.

http://www.gordon-glasgow.org/tirecalc.html

I've been thinking of going to 205/50R17's for my winter boots. They'll give me an overall diameter of 25.07 inches compared to 24.97 inches with the 225/45/17's. They'll work on a 7.5 inch rim and should bite a little better in the snow.

What do y'all think?

Cheers, Phun

 

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in the owners manual it states that when changing tires to stick with ones that are exactly the same diameter. i'm curious to know exactly how much clearance the STi has between the tire and car when the suspension is totally compressed? and if going with a tire that is even slightly taller would cause clearance issues. there can't be too much room for error on this one. the manual also says not to run chains. i'm guessing things are pretty close in those wheelwells.
 

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crashsti said:
in the owners manual it states that when changing tires to stick with ones that are exactly the same diameter. i'm curious to know exactly how much clearance the STi has between the tire and car when the suspension is totally compressed? and if going with a tire that is even slightly taller would cause clearance issues. there can't be too much room for error on this one. the manual also says not to run chains. i'm guessing things are pretty close in those wheelwells.
I think someone here said they used some 255 40 17 or 255 45 17 at the track without any probs, and those would have about the same overall diameter as those skiny snow tires.
 

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Mass insurance

Neisius said:
crashsti said:
in the owners manual it states that when changing tires to stick with ones that are exactly the same diameter. i'm curious to know exactly how much clearance the STi has between the tire and car when the suspension is totally compressed? and if going with a tire that is even slightly taller would cause clearance issues. there can't be too much room for error on this one. the manual also says not to run chains. i'm guessing things are pretty close in those wheelwells.
I think someone here said they used some 255 40 17 or 255 45 17 at the track without any probs, and those would have about the same overall diameter as those skiny snow tires.
I don't believe any 255/40/17 on stock size rims will fit. I use 245/40/17Hoosiers and there is less than 1/4" gap left between the inside wall of the rear tires and the rear strutbody! Fronts have much more room, tough...
 

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I don't believe any 255/40/17 on stock size rims will fit. I use 245/40/17Hoosiers and there is less than 1/4" gap left between the inside wall of the rear tires and the rear strutbody! Fronts have much more room, tough...[/quote]
I can look up who was using them but I know it was 255 and he autoxed with them with out any probs.
 
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