tire width, rim offset, can it be done - Subaru WRX Forum
 
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post #1 of 9 Old 11-07-2010, 04:52 PM Thread Starter
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tire width, rim offset, can it be done

ok i never read up on wheels and tires. i know there is a ton of math involved but i have looked at the tire calculator online as tire size goes. not sure about offset however. plan is to go light light light on the wheel and hopefully the tire too but not a must. now for my 09 wrx the 17" wheel comes with a 225/45/17 not sure on offset. i want to go wider. either a 235 or a 245 tire.

im going with the Bridgestone Potenza RE-11's. they come in a 245/40/17 and a 235/45/17. the 245/40/17 is 24.7" and the 235/45/17 is 25.3". only thing i dont know about is will they fit on the wheels i picked being one is a 17x7 and the other is a 17x7.5?? and of course will it fit on my car?? i am stock suspension now but will be going with the RCE Yellows and shocks at some point. so its a very small drop. my choices for wheels are...

Kosei K1 TS 17x7, 50mm offset, 5.95" back spacing 14.6 lbs
Kosei K1 TS Light Grey Painted

Enkei RPF-1 17x7.5, +48mm offset, 6.10" back spacing 15.2 lbs
Enkei Racing RPF1 Black Painted


PS i can NOT find the Kosei in a different size!! the Enkei's i can find all the way to 10" wide. i have heard that i need at least a 8" to run the 245 tire. but what about the 235 on a 7.5? the Enkeis come in a 35 or 45 offset in a 17x8. what do i need?

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post #2 of 9 Old 11-07-2010, 11:11 PM
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You can run the Enkei 17x8 +45 with no issues, even lowered. I run them on stock height (still kicking around the idea of lowering the car). Im not sure on the newer WRXs the OEM offset was around +53 if I recall. Of course you want to stay as close to that as possible of course. Mosc will be the guy to give you ALL the info, as he really knows his stuff about this.

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post #3 of 9 Old 11-08-2010, 12:01 AM Thread Starter
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ya i found a guy that said the same thing, 17x8 45mm would work with the 245/40/17 with no issues. he runs a 17x9 42mm 245/40/17 and everythings good. not sure if hes lowered or not. it sure looks like it.

the only thing that sucks is tirerack doesnt sell the 17x8 in the Enkei or the lighter cheaper Kosei. so im not sure if i can run the 245/40/17 on the Enkei 17x7.5. i know now im not supposed to run a 245 on a 7" wheel.

if im correct i may only be able to run a 245 on a 8" wheel, a 235 on a 7.5" wheel and a 225 on a 7" wheel.

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post #4 of 9 Old 11-08-2010, 11:52 AM
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Quote:
Originally Posted by synolimit View Post
if im correct i may only be able to run a 245 on a 8" wheel, a 235 on a 7.5" wheel and a 225 on a 7" wheel.
This is not correct. Widths depend on sidewall.
225/45R17 is a '45' sidewall so it handles a variety of wheel widths. 7-8.5"
235/40R17 is a '40' sidewall so it requires a wider wheel. As such, 8-9"
245/40R17 you can fit on an 8" wide wheel... but it's rather floppy. 8-9.5"

The width of a tire relative to it's wheel is determined by sidewall. Here's a rough chart:
Quote:
(these are approximate and estimated from recommended tire sizes of modern STIFF sidewall tires)
50+ sidewall: 5-50% wider tire than rim
45 sidewall: 2.5-30% wider tire than rim
35 or 40 sidewall: 0-20% wider tire than rim
30 sidewall 5-20% wider tire than rim
Wheel / Tire FAQ


I would recommend 235/40R17 for the RPF-1, especially because 245 wide at +45 might cause problems with the rear fender but 235 should be fine. Additionally, 245 on an 8" wheel is a bit floppy. Also the gearing advantage of 235/40R17 is beneficial (also closer to stock).

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post #5 of 9 Old 11-08-2010, 12:02 PM
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Mosc to the rescue. I had the wife find a pic of my tires on my car for the size. I run a 235/40R17 on my.

Joshua
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post #6 of 9 Old 11-08-2010, 12:30 PM Thread Starter
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Quote:
Originally Posted by mosc View Post

I would recommend 235/40R17 for the RPF-1, especially because 245 wide at +45 might cause problems with the rear fender but 235 should be fine. Additionally, 245 on an 8" wheel is a bit floppy. Also the gearing advantage of 235/40R17 is beneficial (also closer to stock).
and by saying that are you meaning the RPF-1 from tirerack that is 17x7.5 48mm offset?? or the 17x8 45mm offset? im dumb and just saw they have a drop down bar so you can pick a 7.5" or a 8" wheel.

they also have a 225/45/17, 235/40/17, 235/45/17, and 245/40/17 for the RE-11's

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post #7 of 9 Old 11-08-2010, 01:57 PM
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235/40R17 requires 8" minimum. I'm assuming you're going with the 17x8" +45 wheel. 235/40R17 RE-11's will be a ####ton of grip for the street and beyond. The gearing will be good for your car, as will the ratio of wheel to tire (good steering response).

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post #8 of 9 Old 11-08-2010, 05:58 PM Thread Starter
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then thats what ill go with.

i was hoping to get closer to the 25" size of my stock tire because the 235/45/17 is 25.3" vs. the 235/40/17 which is 24.4" but i like the idea of a lower pro tire giving more grip or sidewall support on hard turns.

but that brings up another question. since im a 24.4" now what negative or positive results will i see with acceleration, particularly drag racing?

o ya one more thing. stock rim is 23.5lbs, whats the tire weigh? anyone know the weight of a RE-11 235/40/17?

EDIT NVM i found both, -6.9lbs a wheel should be nice!

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post #9 of 9 Old 11-09-2010, 01:07 AM
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Remember the 06-07 WRX? Same engine, virtually the same driveline, came with 24.6" tall stock tires. Just saying, stock WRX size is a relative term.

Acceleration will be helped by a shorter ratio, as will steering response thanks to a stiff 40 sidewall tire.

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