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I've decided to go with 18" wheels.

First, as I understand it, as long as I stay with 7.5" width and 225s I will not have to toy with the offset. Is that a correct assumption?

Second, if I go with a 50 profile then that will fill the wheel well profile more but won't that effect the speedo/odometer? If so, can the dealer do a simple reprogramming?

Since the Sport Appearance package comes with 18x7.5 and 50 profile tires I am assuming that I can do this without having to make any wheel well mods. Is that a reasonable assumption?

Sorry for the simple questions but I have no real experience with this and after two nights of searching the threads I thought I'd just asked the dumb questions and get them over with.

Thanks guys, I really am enjoying the forum.
 

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Just for your info , I put on new 18" Karezzas with 225/45/18 Dunlop Direzza tires . I did a mileage run before I put the new rims on and got 9.8 miles , I did the same run after the rims were on and got 9.0 miles . So for every 100 miles I travel the speedometer with register only 90. A moral dilemma !!! Rich
doorknob said:
I've decided to go with 18" wheels.

First, as I understand it, as long as I stay with 7.5" width and 225s I will not have to toy with the offset. Is that a correct assumption?

Second, if I go with a 50 profile then that will fill the wheel well profile more but won't that effect the speedo/odometer? If so, can the dealer do a simple reprogramming?

Since the Sport Appearance package comes with 18x7.5 and 50 profile tires I am assuming that I can do this without having to make any wheel well mods. Is that a reasonable assumption?

Sorry for the simple questions but I have no real experience with this and after two nights of searching the threads I thought I'd just asked the dumb questions and get them over with.

Thanks guys, I really am enjoying the forum.
 

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2006 Black 3.6L CTS
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doorknob said:
Second, if I go with a 50 profile then that will fill the wheel well profile more but won't that effect the speedo/odometer? If so, can the dealer do a simple reprogramming?
What difference does profile make? Some please explain the quantitative values such as "50 profile". I'm also looking to upgrade from the stock 16'' wheels to AT Italia Inox 18'' wheels. But I'm afraid that it might affect my odometer readings :( . I'm sure that odometer effects are dependant on the tires, not the wheels. Someone tell me for sure.
 

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OK lets see is I can help. Take this size as an example



245-50-16



245 refers to the width of the tire. (the contact patch if you will) It means the width is 245 MM or 24.5cm



the 50 is the profile. It is not a constant. It is a percentage of the width. So in this case the sidewall so 50% of 245. If the tire was 245-45-16 then the profile would be 45% of 245. Make sense?



And of course the last number reffers to the diameter of the rim. 16 inches in this case.



Whenever you change rims for a larger size you want to do something called "Plus size"



The overall diameter of the tire should be the same. The only differences you have more rim and less profile.



If you do not maintain the basic over all diameter, your speedo will read wrong and you can run into other problems like ABS issues.



Hope that helps, Jason
 

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