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Discussion Starter · #1 ·
I've been on here for years and never dealt with an "offset" issue for my MR2. However, my other vehicle is a 2009 GMC Sierra 1500 and I am looking to get some aftermarket wheels/tires on that. The problem lies in that one seller is telling me the "offset" won't work with the type of wheels I would like to buy, while another seller is telling me that they will fit fine.

Here is what the seller who said they won't fit said: "These wheels will fit your vehicle correctly, but because of the +12mm offset, these wheels will extend away from the center of your vehicle (towards the direction of the fender) about 0.5 - 0.75 inch more than your stock wheels. The a stance that is similar to stock, you want a wheel with a +30mm offset. These wheels are currently only available in the +12mm offset to fit your vehicle."

Here is what the seller who said they would fit said: "
Forget compatibility charts. They don't work. LOL. 18" RT6's come in 0 offset only which should work fine on your truck. They'll definitely fit. You can put up to 26" on those babies. "

And here are the wheels in question: http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=280819252964&ssPageName=STRK:MESE:IT

To any of you wheel/offset masters....can you answer the question once and for all....will these wheels fit okay on my 2009 GMC Sierra 1500? If there is any other information needed to solve this puzzle I will be happy to try and find it.
 

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Go outside and measure how much more room you have between the wheel and fender horizontally. That is how much more offset you can "add" before touching the fender. Offset number will go lower when you "add" until zero then it will go negative. This is keeping with the fact that the new wheels are the same width as stock. If wider more calculations are needed.
 

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A quick look at some GM full size forums tells me that your wheels are likely +30 or 31, so the first seller was right about that. The second seller is also mentioning 26's, which has nothing to do with offset and makes me question whether he knows what he's talking about, unless he's just illustrating how much wheel well you have to play with. Anyway...

In case you're not totally in the know with this, offset is just the measurement in millimeters from the hub (where the wheel attaches to your vehicle) to the true middle of the rim. As has been said, it's a scale that spans both positive and negative, where 0 offset is a wheel with a hub face at the exact center of the wheel. Going toward negative numbers effectively moves the outside edge of the wheel away from the car, and going toward positive numbers moves the outside edge toward the car. The difference between the old and new figures is the number of millimeters the outside edge has moved.

So, now we can do the math pretty easily. You're starting at +30 and you're moving to 0. If the new wheels are the same width as your current wheels, that's 30 millimeters further away from from the car, or about 1.2". I'll bet dollars to donuts that your current wheels are further inside the fenders than that. Probably twice that much or more. But, you also have to account for increased wheel width. Specifically, for every inch of wheel width you're adding, we can add another 13mm to that offset change to get an idea of where the outside of the new wheel is. I think your stock wheels are 17x7.5", so we need to add that extra 13 for these new 8.5" wide wheels. That's a total change of 30+13mm, or effectively 1.75" further out. So, that's the change you need to be comfortable with. I can't tell you for sure if that's going to work since I don't own a 1500, but I'd say it will be just fine unless you're planning to add some crazy 40+" tires. What I can say for sure is that there's no specific reason it won't work from a theoretical perspective.
 

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Discussion Starter · #4 ·
Thank you midshipman01, that was the most well informed, perfectly groomed, and reader friendly response I have ever seen in any forum on spyderchat. Everything was computed perfectly and well explained. I think I will like the way these will fit, give the truck a more aggressive stance. I had the exact same wheel on my 2005 Sierra 1500 and the wheel/tire stuck out from the fender a bit, it looked great. This time I am planning to upgrade and get the wheel in size 18 and some Yokohama Geolander AT-S tires to go with them. I plan to buy both the wheels and tires from Ebay.
 
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