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Old 11-30-2007, 01:28 PM   #1
blkboxster
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Tire size question

So 225 40 18, and 245 40 18 whats the difference , i mean was going to get 245 40 18 up front for my new tires, and jeph said there to big which i agree with, but will 225 40 18, fit my new rim?? Man i have a fealing no one will understand this thread,
i just dont want to screw up mt purchase when i buy my tires.

thanks

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Old 11-30-2007, 02:35 PM   #2
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Let me take a swing at your questions.

The difference between a 225 and 245 tire is the height of the sidewall. A 245 will be taller. Whether either will work will be influenced by the width of the wheel you use. I remember seeing your thread about new wheels, but I forgot what width and offset your fronts are.

As an example, say your fronts are 8.0" wide and 50mm offset (typical Boxster wheel from Wheel Dynamics). The 225 may have to "stretch" to fit the wheel, which will cause the sidewall height to be smaller than if it was on the standard Porsche 7.5" wide 18" wheel. A 245 will work on the 8.0"/50mm wheel - at least it does on the Carrera S wheels I use with my summer tires on my '01 S (I have the winters on now so my sig pic has changed). BUT, there is very little clearance between the tire and the shock spring, however it is a static distance and not a problem. Don't know if there is a difference between the shocks on a base and S Box - specifically where the bottom of the spring is on the shock. One thing I noticed with the 245 is that the height of the sidewall makes the front of my Box look high. If I had it to do over, I would go 235 and get the best of both worlds - proper sidewall height and no tight clearance issues.

My last paragraph mentions offset but doesn't get into the details. If the offset is not 50mm, a different offset may move the wheel closer to your shock and increase the possibility of rubbing issues. A shorter sidewall tire may alleviate the problem but I don't have first hand experience with a different offset.

Hope this helps. You are wise to ask before making an expensive mistake. I'm sure others can chime in with their thoughts. If I had to error one way or another, the 225 will work while the 245 should work.
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Old 11-30-2007, 02:40 PM   #3
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Just to expand on tire reading basics:

225 40 18

225 = section width of the tire, in millimeters

40 = aspect ratio - the percentage of the section width that then defines the sidewall height. In this case, the side wall height is 40% of the tires width (225 x .40 = 90mm)

18 = rim diameter in inches

so.. a 225-40-18 is both narrower (225<245) and "shorter" ( 40% of 225 =90, 40% of 245 = 98) than a 245-40-18.
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Old 11-30-2007, 02:41 PM   #4
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Actually the 225 and 245 are the width of the tire.

Tire sizes work like this

225/40 - 18

tire width (millimeters)/tire sidewall height (% of width) - wheel diameter (inches)

So a 225/40 is 20mm narrower than a 245/40.

Each tire has a sidewall 40% of the width.

So 225/40 = 90mm sidewall height
and 245/40 = 98mm sidewall height

HERE is a link with more explanation, or just google 'tire size explanation'
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Old 11-30-2007, 02:47 PM   #5
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Heres the size

Size: 18x8 front/ 18x10 REAR

Offset: +48mm front/ +40MM REAR
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Old 11-30-2007, 03:04 PM   #6
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I have the 18" factory option turbo twist rims on my S. I just ordered a new set of Bridgestone 960AS Pole Positions from TireRack in the following sizes:

F: 225/40-18
R: 265/35-18

As you can see the sidewall height number drops down when using a wider tire. But if you do the math, the sidewalls are less than 3mm variance since that number is actually a ratio.

Hope that helps!
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Old 11-30-2007, 03:06 PM   #7
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Quote:
Originally Posted by schwank
I have the 18" factory option turbo twist rims on my S. I just ordered a new set of Bridgestone 960AS Pole Positions from TireRack in the following sizes:

F: 225/40-18
R: 265/35-18

As you can see the sidewall height number drops down when using a wider tire. But if you do the math, the sidewalls are less than 3mm variance since that number is actually a ratio.

Hope that helps!

So will F 225 40 18
R 265 35 18 which i think im going to order will fit my rims??
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Old 11-30-2007, 03:11 PM   #8
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ET40 on the rear may cause some outside rubbing if you go wide on the tire. You may need to increase negative camber to offset or roll the fenders. My rears are ET47 with 275/40/18 with neg 2.5* camber.
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Old 11-30-2007, 03:15 PM   #9
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Quote:
Originally Posted by jmatta
ET40 on the rear may cause some outside rubbing if you go wide on the tire. You may need to increase negative camber to offset or roll the fenders. My rears are ET47 with 275/40/18 with neg 2.5* camber.
So can you give me the new tire sizes for the front and rear, what you said just confused me.
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Old 11-30-2007, 03:21 PM   #10
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Quote:
Originally Posted by blkboxster
So can you give me the new tire sizes for the front and rear, what you said just confused me.
For your rear rim being so wide, so smallest width you can use that will stretch and fit is 265/35. If you increase the width (275-285) then you will have to decrese the sidewall hieght (275/30 or 285/25). But as I stated before if you go much wider in the rear you will have fender/strut rub issues. Your front rims are the same width as stock rims so there is no difference there. 225/40 front will fit fine and fill out the wheel well without spacers.

Front 225/40
Rear 265/35 or 275/30.
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Old 11-30-2007, 04:46 PM   #11
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N rated Tire?

I just left my 986 at the local dealer for a 45K service. my tires are shot and I have been shopping for a new set of 'shoes'. I was discussing having the tires shipped from the Tirerack for installation with a brake service and was advised that I needed tires with an 'N' rating approved by Porsche??? I was told that there are 'Pilot Sports' and then there are 'N rated Pilot Sports". Does anyone know if this has any merit... or is my local dealer just trying to jack me for a mark-up on 'their tires'?

Last edited by Avidhts1; 11-30-2007 at 04:50 PM.
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Old 11-30-2007, 05:12 PM   #12
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Quote:
Originally Posted by Avidhts1
I just left my 986 at the local dealer for a 45K service. my tires are shot and I have been shopping for a new set of 'shoes'. I was discussing having the tires shipped from the Tirerack for installation with a brake service and was advised that I needed tires with an 'N' rating approved by Porsche??? I was told that there are 'Pilot Sports' and then there are 'N rated Pilot Sports". Does anyone know if this has any merit... or is my local dealer just trying to jack me for a mark-up on 'their tires'?
There are "N" rated tires. They meet Porsche OEM specs. While I don't know the specific differences in the Boxster application, I do know that on the new 997 GT3, for example, the "N" spec Michelin has different wear characteristics (they don't were out as fast) as the Non N spec Michelin tire.

That said, Porsche usually specifies more than one brand/tire as "N" spec.. in the past, Michelin, Continental, Pirelli and Yokohama have been OEM spec tires. I doubt you would notice the difference in the driving between an "N" and "non-N" spec of the same brand/model of tire.
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Old 11-30-2007, 06:03 PM   #13
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re: 'N' rated tires > porsche recommended OEM brands

Many thanks for the feedback- My '03 986 came clad with Pilot Sports... No qualms but no point of reference either. I've been perusing the inventory at the Tire-Rack and the most suitable candidates seem to be the OEM recommendations,... the 'Conti's", Yokohama's, (Pirelli's didn't get rave reviews) and... the Dunlop's... The latter seems to be a very good value and have received reasonable feedback. Does anyone have any input based on a little more personal experience than I have, a 'Noobie 986 owner'

Last edited by Avidhts1; 11-30-2007 at 06:08 PM.
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Old 11-30-2007, 06:52 PM   #14
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Thanks for hijacking the thread
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Old 11-30-2007, 06:59 PM   #15
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re: hijacking the thread

Sorry- Didn't realize this was such a specialized discussion. my forgiveness if I corrupted your direction
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Old 11-30-2007, 07:06 PM   #16
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Originally Posted by Avidhts1
Sorry- Didn't realize this was such a specialized discussion. my forgiveness if I corrupted your direction
No worries ,ha
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Old 11-30-2007, 07:08 PM   #17
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N rated Tire?

Thanks 'racer d'- The dealer referenced the 'N' rating as having to do with side wall stiffness/thickness or the rigidity of the tire... He said that without this rating the car would handle like S**t... Now I feel like a total dweeb.
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Old 11-30-2007, 08:17 PM   #18
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about to order tire but what does Xl mean at the end ? only on front tires

225/40HR18XL front

265/35HR18 rear



thanks
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Old 12-01-2007, 12:28 PM   #19
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XL means extra load or reinforced sidewall...not really sure of it's applicability other than to differentiate from standard load tires. Probably has to do with the higher speed rating and the fact the fronts handle the turn-in..

Last edited by jmatta; 12-01-2007 at 12:34 PM.
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Old 12-01-2007, 02:03 PM   #20
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Quote:
Originally Posted by jmatta
XL means extra load or reinforced sidewall...not really sure of it's applicability other than to differentiate from standard load tires. Probably has to do with the higher speed rating and the fact the fronts handle the turn-in..

thanks.....


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