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'04 CTS
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Discussion Starter #1
Since I’m eventually going to put 18 inch rims on my ‘04 Lux, I figured I’d see what rim width and offset I needed before I went shopping. To do that I first had to decide what tire size I was interested in. I’ve changed the suspension to FE2 spec so I thought 245s would be a good combination of looks and performance. To have a decent selection and so as not to alter the speedometer, I chose a 40 profile. The price of a set of 18 inch tires is already WELL above that of my 16s, if I didn’t draw the line somewhere I could end up with a $900 tire purchase, so I decided on Pirelli PZero Nero M&S in size 245/40ZR18 to use as source of specifications.

The stock Goodyear RSA - 225/55HR16 had a section width of 9.1 inches on 7 inch rims so without crawling under the front of the car and guessing at how far I could push the tire clearance envelope, I decided to take the original tire spec as my target clearance measurement. As with most 245/40-18 tires, the Pirellis would go on a rim as narrow as 8 inches but the section width spec was 9.6 inches on an 8.5 inch rim. According to Tire Rack, that meant a section width of about 9.4 inches on an 8 inch rim.

I just had to shop for an offset spec that allowed for half the change in section width. The stock rim has a 51mm offset so for an 18X8.5 inch rim the offset would need to be .25 inches less than 51… call it 44mm. For an 18X8 inch rim the offset would need to be .15 inches less, or about 47mm.

Smaller offsets than these provide even greater clearance but make the wheels stick out further. One wheel which I like is 8 inches wide with a 42mm offset, so the rim would stick out .85 inches further than the 7 inch wide stocker. With the long wheelbase of the CTS, the extra width wouldn’t seem to be an issue for either appearance or performance.

The only thing that still bothers me about this change in wheels is the unsprung weight. The stock combo is a little less than 41 lbs, 19 for the rim and 22 for the tire. The 18X8inch/42mm wheel weighs 26 lbs and the Pirelli weighs another 26 lbs. That’s 11 pounds more per wheel than stock!

I know… I’m WAY too anal. If there's something wrong with my thinking here then it would be nice to know it now, so all comments and criticism are gratefully accepted.

Strud
 

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'04 CTS
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Discussion Starter #2
OK, I’ve reread this and it seems that, as usual, I had my head up my butt last time. Here is the corrected version.

Since I’m eventually going to put 18 inch rims on my ‘04 Lux, I figured I’d see what rim width and offset I needed before I went shopping. To do that I first had to decide what tire size I was interested in. I’ve changed the suspension to FE2 spec so I thought 245s would be a good combination of looks and performance. To have a decent selection and so as not to alter the speedometer, I chose a 45 profile. The price of a set of 18 inch tires is already WELL above that of my 16s, if I didn’t draw the line somewhere I could end up with a $900 tire purchase, so I decided on Pirelli PZero Nero M&S in size 245/45ZR18 to use as source of specifications.

The stock Goodyear RSA - 225/55HR16 had a section width of 9.1 inches on 7 inch rims so without crawling under the front of the car and guessing at how far I could push the tire clearance envelope, I decided to take the original tire spec as my target clearance measurement. As with most 245/45-18 tires, the Pirellis would go on a rim as narrow as 7.5 inches but the section width spec was 9.6 inches on an 8 inch rim.

I just had to shop for an offset spec that allowed for half the change in section width. The stock rim has a 51mm offset so for an 18X8 inch rim the offset would need to be .25 inches less than 51, or about 44mm.

Smaller offsets than these provide even greater clearance but make the wheels stick out further. One wheel which I like is 8 inches wide with a 42mm offset, so the rim would stick out .85 inches further than the 7 inch wide stocker. With the long wheelbase of the CTS, the extra width wouldn’t seem to be an issue for either appearance or performance.

The only thing that still bothers me about this change in wheels is the unsprung weight. The stock combo is a little less than 41 lbs, 19 for the rim and 22 for the tire. The 18X8inch/42mm wheel weighs 26 lbs and the Pirelli weighs another 28 lbs. That’s 13 pounds more per wheel than stock!

All comments and criticism are gratefully accepted.

Strud
 

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Black the Darkside
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22,415 Posts
Your post was to long to read it all.:eek:
What you need is a 8" wide rim. Off-Set about 40mm. 245x45x18" tire. Look for a VR rating if you intend to drive fast. Pirelli makes them.

18" was a wise choice.:thumbsup:
 

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2003 CTS Manual Trans., '93 STS
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3,035 Posts
You're definitely thinking in the right terms. This has been discussed a million times before, so here's a few things to note:

The stock V (exact same chassis) runs 245/45R18's on 18x8.5" wheels with 42mm offset. While not changing the overall diameter by sticking with 40 series tires sounds good in theory. Several of us have checked with GPS and found that the stock tires are too small and over report your speed. The V sized tires put the speedo exactly on what the GPS reads. They also fill the wheel well better than a 40 series tire. They have the same sidewall as the stock 225/50R17 sport tire, just adding 1" to the wheel (and corresponding overall tire) diameter.

With that said, you won't likely find a 42mm offset wheel easily. I've seen lots of 40mm, a few 44 or 45 mm. Most of us running 18's are running 245s on 18x8 wheels with 40mm offset. They fit perfectly with no rub (as expected). Going to a 40 or 45 series tire is purely a personal choice, but I always thought the stock CTS tires were too small for the wheel wells. Anything less than about 45mm offset with a 245 tire will rub the upper A-arm pinch bolt, up front.

I would recommend finding the proper 5 on 115mm lug pattern, although plenty are using the popular 5 on 4.5" (114.3mm) lug pattern with little issue, other than possibly running hubcentric rings.

My 18x8" wheels are 23 lbs each, each with at total weight of 43 lbs with tire mounted and aired to 32psi. Although Mille Miglia doesn't make the Evo 5 any more, there's a Chinese knockoff (Sport Edition KV5) at Tire rack with the same style and specs.
 

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'04 CTS
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43 Posts
Discussion Starter #5
Robert,

I agree! You’re one of the guys that got me started on this quest.

~~~~~~~~~~~~~~~~~~~~~~~~~~~
Odysseus,

Yorktown? You’re practically a neighbor.

Thanks for the info. I know what you mean about the speedo. I put mine against GPS and it reads about 1.5% high at 75 mph but that’s quite a way from the 3.5% difference that a 45 profile would reduce the indicated speed. So when I looked at the CTS-V specs for info, I just figured that Cadillac recalibrated the speedo to the larger diameter tire. From what you’re saying, that’s not true. The 45 profile would make my speedo read 2% low though and, especially with Virginia’s new civil penalties law, that’s the wrong way to have an error.

The Mille Miglia Evo 5 is a nice looking wheel. I’m looking at the Ronal R38 w/polished lip, 18X8, 5X115, 42mm offset according to the distributor although their web site [www.ronalusa.com/wheels/r38.html] suggests 40. I have respect for Ronal's quality but, unfortunately, they’re 26 pounds.

I’m curious, what tires are you using that weigh 20 pounds?

Strud
 

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2006 CTS 2.8L
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2,471 Posts
I went with 18x8 rims on 245x24xR18 tires and never looked back. Also, my car is lowered and has yet to have problems.
 
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