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I just bought a new set of wheels and tires and was surprised and disappointed when I got them to the tire shop and was told they wouldn’t fit. I’m looking to get opinions on these measurements.
The wheels are 21in diameter and 10.5in wide with a 42mm offset. The tires are 265/35/21.
I thought this was a conservative fit based on what others are running and never considered it wouldn’t fit. I’ve seen 10.5in 22s on much meatier tires.
The tech at the shop told me that there was about 0.5” of clearance to the inner of wheel well while the car was up in the air and that would go to practically zero clearance when the weight of the car was sitting on the suspension. He said to come back with 10” wheels.
Does anyone have an opinion on where I’ve gone wrong here? Does the air suspension rise up when you turn these cars on? Maybe that’s what the tech is missing?
Std 20" wheels are 20×8.5 36mm offset front 20×9.5 43.5mm offset rear
For the outside edge of the tire to remain constant and with the change from 9.5 to 10.5 the offset should have gone from 43.5mm to 68.9mm = 43.5+25.4
Since the factory is <let's just say> conservative WRT outside versus fender, you should have been able to take 15mm off that to get 53.9mm.
This still leaves you 11-12mm to close to the fender. 12mm, BTW, is ~0.5"
Now, you went down from 275/40/20 to 265/35/21, losing 10mm of footprint width, this saves you 5mm (on both sides).
Yep. I can see why the tire people said what they did.
Offset does not stay constant as wheel width increases.
Thanks for the feedback, Mitch. I might be misunderstanding but shouldn't I have been reducing offset (rather than increasing it) to keep the backside of the wider wheel in the same position relative to the fender liner? Also, I didn't think I'd need to go inch for inch in reducing offset as I went wider because half of that increase in width would be on the front side of the wheel. If I'm understanding this correctly, I should have reduced offset by half the increase in width (i.e. made a 12.7mm reduction to move to a +30mm offset) instead of increasing it by 25.4mm. That's if I wanted to keep a stock distance.
I plugged the stock measurements you shared into a calculator and found that I was only bringing the inside of the wheel 11.7mm closer to the car. I am on the same page as you that I expected at least 15mm of room relative to the stock setup.
I think I'm going to swap out the rears for 10" +30 offset and I think that will keep the back of the wheel in the same location as a stock setup but push the outer face out half an inch.
Does that seem reasonable? I could very well be wrong about all of this because I got myself into this mess in the first place.
I'd still love to hear from people who are running 10.5" or 11" rears. I feel like I'm admitting defeat going to 10s because I know people are out there.
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