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04 CTS 3.6L "The Gray Ghost"(R.I.P), 05 CTS 3.6L "Fin"
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Discussion Starter #1
Hey CTS brethren,
I have stock 17's and am going to buy new rims. I want to go 18".
can I just put 18's on, or is there some sort of reconfig to the computer that is needed to keep my MPG the same and speedometer working the same?
A bigger wheel is going to have less revolutions per mile, right?
Which would make my speedometer read wrong, right?

Or, can I just get a lower profile tire?
Thanks for any feedback
 

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Go with the same size as the CTS V and I believe that you should be fine. (Some have posted that the V speedo is accurate, while the non-V models are the ones that are out of sync. I can't answer that though. Nor would you want me to as I have 18 on the front, and 19 on the rear...)
 

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They should make a new model name for mine but I guess a CTS
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all that matters is overall tire diameter but just so you know the dealership can resync the car for you
 

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Cadillac CTS
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The proper size for non-V series is 245/40 r18, otherwise you will be going faster than the speedo indicates. 40series tires will drive a little more harsh than 45series tires though.
 

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05 CTS
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I have 245/45/18's on mine...and I don't notice any difference in the speedo. I don't believe it's affected at all personally..

But..I'd have to ask a cop... tee hee...
 

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04 CTS 3.6L "The Gray Ghost"(R.I.P), 05 CTS 3.6L "Fin"
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369 Posts
Discussion Starter #7
Thanks for everyone's responses.

Go with the same size as the CTS V and I believe that you should be fine. (Some have posted that the V speedo is accurate, while the non-V models are the ones that are out of sync. I can't answer that though. Nor would you want me to as I have 18 on the front, and 19 on the rear...)

03cts,
are you saying that as my car is now, the speedo is not accurate and if I put 18's on, like a v has, then my speedo will read correct? Not saying your wrong, but I find it hard to believe that the factory would ship cars programed for a certain size tire and put a smaller tire on the car.
Then again, that might be why the auto industry is in so much trouble.

If I do have to have the dealer reprogram the car, any idea how much something like that cost? I'm sure it is a samll job for them, but dealing with computers (I'm an IT guy), I know how things can be overcharged.

Thanks again for everyones input.
 

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04 CTS 3.6L "The Gray Ghost"(R.I.P), 05 CTS 3.6L "Fin"
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369 Posts
Discussion Starter #8
As you can prob tell, I don't know a whole lot aboout tires and wheels.
I just looked at my tires again and the size is 225/50/zr17. I do know that the zr17 means 17", however, I measured the rim itself and it is 18". I'm getting so confused. Does someone have a link or something for a quick, or in depth tire and wheel tutorial?
 

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2005 Cadillac CTS Every Option
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516 Posts
If you measure the actual diameter of the rim, from outside lip to outside lip, it will usually be .5 to 1 inch bigger than rated. It is still a 17" rim. Dont worry about anything other than the diameter of the tire. Your stock setup is calibrated right with the computer, so if your new tires are the same diameter as the old ones, it wont need re calibrating.
 

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Thanks for everyone's responses.




03cts,
are you saying that as my car is now, the speedo is not accurate and if I put 18's on, like a v has, then my speedo will read correct? Not saying your wrong, but I find it hard to believe that the factory would ship cars programed for a certain size tire and put a smaller tire on the car.
Then again, that might be why the auto industry is in so much trouble.

If I do have to have the dealer reprogram the car, any idea how much something like that cost? I'm sure it is a samll job for them, but dealing with computers (I'm an IT guy), I know how things can be overcharged.

Thanks again for everyones input.

It has been mentioned before here that the V speedo might be the only one that is accurate. I have no idea whether or not that statement is true. If you are starting with an accurate speedo, what matters (as suggested by others) is the diameter of the tire and not the wheel. If you put on a bigger diameter, your speedo will show you going slower than you truly are (translation: tickets. However, I suppose that your odometer won't show as many miles:bouncy:) Keep in mind though, we are likely only talking miniscule %'s.
You also have the concern about tire width and offset which can result in tire rubbing if you're not careful. Also the lower the tire profile, the rougher the ride. (In general, the V wheel and tire size is a nice model to copy.)
I found the www.tirerack.com website to be very helpful as it shows tire diameters (as well as being able to model different size wheels on the car.)
Good luck. There are tons of threads in this forum that have dealt with this since 2003!
 

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I went from 17s to 18s with a bigger overall diameter and my car goes faster than the spedo indicates compared to before.

I know because I can see my GPS speed at the same time.

Typically, you should stay within 3% of the diameter of your original setup - bigger diameter is harder to brake with. I read this standard from a wheel shop website.
 

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04 CTS 3.6L "The Gray Ghost"(R.I.P), 05 CTS 3.6L "Fin"
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Discussion Starter #12 (Edited)
thanks guys, your all the best. This is a GREAT site.
I found this tire calculator that gives differences for different tire sizes:
Sidewall Radius Diameter Circumference Revs/Mile Difference.
Don't haze me cause it's a mazda site. It does what I need it to and I think others in my situation might find it very helpful.

http://www.miata.net/garage/tirecalc.html

Thanks for the input and knowledge.

I put in my stock 17" tire size (225/50/17) and found 225/45/18 is only .1" in diameter bigger, with only two revs/mile more.
I don't think that will throw of the speedo much. Might make me go like 1/4-1/2 mile an hour faster. That sound about right?
 

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03 CTS 3.2L (Sold) / 09 CBR 600rr /2014 GMC Sierra
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Just buy 20z....don't worry about your speedo
 
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