20" wheels difference in offset..40mm to 18mm - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 05-02-2009, 02:00 AM   #1
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Default 20" wheels difference in offset..40mm to 18mm

Ok, I posted a thread earlier asking about the widest wheel one could use on the bonneville. every bit of research I have done tonight says that the 9" wide wheel will work. My problem is, the wheels I REALLY want only have an 18MM offset which when I look at most aftermarket 20" wheels for the bonneville they are 40mm offset. So, here lies the question, will it still work at 18mm offset? I used an offset calculator and it said the wheel would be 1mm further away from strut, but would stick out 1.61 inches more than the factory wheel. Please, if anyone understands my questions and knows the answers, please chime in and help me out here before I jump. Help me if you can. Thank you. J
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Old 05-02-2009, 09:49 AM   #2
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The larger the offset figure, the more it protrudes into the well
when you have a factory wheel at 40mm then go to a wider wheel, you will need a bigger offset for the centerline of the wheel to remain in the same postion

When you start goin outside of the vehicle, the outer part of the tire traveles further out on turns and will most likely hit the inner fender well

So my vote is NO at 18

Another comon over looked issue, is rotational mass and weight
When you go to larger diameter wheels that are heavier, it greatly reduces braking and will freguently make the abs come on in light braking situations
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Old 05-02-2009, 11:36 AM   #3
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Thanks for your reply. I found out it is 24mm offset which is till not much. But when I do the math and convert to inches, it shows the wheel will protrude towards the outer well at 1.37 inches. Is that really enough to cause problems? J
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Old 05-02-2009, 12:04 PM   #4
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Quote:
Originally Posted by immusicman View Post
Thanks for your reply. I found out it is 24mm offset which is till not much. But when I do the math and convert to inches, it shows the wheel will protrude towards the outer well at 1.37 inches. Is that really enough to cause problems? J
Besides answering that question, what size tires would you put on 20'* so they don't rub? J
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Old 05-02-2009, 12:39 PM   #5
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You can draw it out on a big piece of cardboard
draw a line down the center with a center hash mark, that is you hub face
from above the hash, measure from the line out to the right about 40mm [1.5"]
make a mark at the end
slighlty underneath that line measure from the mark back across the line the widht of the factory wheel, about 7.5" and

do the same for the 20'* under the hash, using the 24mm and the wheel width of 9" and you can see the difference

Also note that you would only want 115mm bolt pattern, not 114.3 or 4.5
My opinion is it would be an expensive gamble:crazysmile:

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Old 05-02-2009, 12:41 PM   #6
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You need 20'* that are about the same diameter as factory about 27.3" ?
That should always be a starting point when shopping for bling
Find the tire that matches dia, know what they cost, then mate to appropriate rims

some 255/35/20'* are about 27"
the 275/40/19'* are 27.3 etc

Last edited by GreenLantern; 05-02-2009 at 12:59 PM.
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Old 05-02-2009, 01:02 PM   #7
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Holy crap, i just found the same wheel with a 47mm offet at the 115 bolt pattern and only 8 inches wide! Perfect? Please tell me it is and what size tire should I ask for with it? Tires on the GXP 20" aftermarket (according to tire rack ) is 245/45/20. Thanks!
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