Lotus99

From what little I've come to understand about this stuff... When you end up getting a lower offset for the same width wheel as your current one, it means it will stick out more.

That's confirmed by the Offset Calculator at this site also.

I'm considering upgrading my winters from 17 to 18, and Tire Rack offers up a wheel with a 30 offset. Since I know their stuff is all suggested by computers, it should technically fit.

But is there any reason why it wouldn't? I would guess you wouldn't want to go higher offset than what your car comes with, so the wheel doesn't go inwards and hit stuff, but lower should always be safe, no?

I don't know how to use/read the Backspacing number either. The wheel I'm looking at has 5.7" whereas most of the others with similar offsets to OEM have just over 6".
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JayArras

Just as an FYI, offsets are measured in millimeters. For an xi, the offset should be between 35 - 45 mm. The 30 you mention won't fit. Also, just so you know, the narrower the tire, the better for snow. Generally 17" rims are better for winter because you can get the narrowest tire possible for your bimmer in that size.

Lotus99

You're right about the narrower tire on snow guideline, I'd be going from 225's to 245's.

But that aside, I am wondering why they wouldn't fit, especially since Tire Rack says they do, and I can't imagine with their databases, they'd suggest something that wouldn't fit.

Besides telling me the guideline for Xi cars of 35 to 45, can you actually tell me WHY it wouldn't work?

And if there's a flaw to my logic (that it should be safe to lower offset always, since you're getting more clearance and not less), where it is?

JayArras

It's simple. The higher the offset, the deeper the recess on the wheel. If you try to put a lower offset wheel on your xi it won't sit flush and you won't be able to attach the lugs.

I'm not sure what rims you're referring to, but if Tire Rack is telling you that a 30 mm offset will fit an xi, that's stretching it.

constant

They will fit fine, Tire Rack is right. I run a rear 19x10" ET25 on my wagon in the summer w/ a 255/35 tire. My setup is pushing the limits without fender rolling. A ET30 w/ 8" wide rim is nothing, just take it easy on tire width. Mine: https://5series.net/forums/tires-whe...7/#post1505106

Check out this calculator: http://www.fitmywheels.com/fitimage/.../18/18/8/30/-1
You can see I put in mine vs the size you're considering and can see how much more tuck you'll have than I do.

Hope that helps from someone who's actually been there, done that.

constant

Ps you can look through the thread w/ my rear wheel pic; you'll see tons of other examples of more extreme setups than what you're proposing. It should boost your confidence. I'd skip 18's and go to 19's myself. :-)

Lotus99

Thanks Constant. Good to know. Was hoping that as I start pricing out my replacement 17" winter tires, if the differential isn't much to just getting a full 18" wheel & tire combo, I'd just take the opportunity and upgrade. I'll see how the pricing works out as things go on sale.

But I'm still puzzled by how my logic is wrong, in that going with a lower offset wheel than OEM (all other things being equal) would give you MORE inside clearance, so how could that setup run into the suspension components?

There must be something I'm missing though, as otherwise, all our XI cars wouldn't come with these 40+ offsets...

And to jayarras' point, you need different lugs when you go with different offsets? I thought only when you get spacers do you need longer bolts?
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constant

Your logic is not wrong, but only up to a point. You do create more clearance inside the fender...at the expense of losing clearance on the outer fender. So a lower offset keeps moving the wheel outward until this happens: https://imageshack.com/i/5p1h2otopdawg6dc3j
This is referred to as "poke", which some people like this look.

See the problem here? Hit a big bump, and your tire could wreck your metal fender as it travels upward.

You do not need new lug bolts unless you run spacers or the wheel has an odd seat profile for the mounting holes.

Cpalacay

Lotus, it looks like Constant has summed up everything pretty well. As long as 18"s are the biggest size you plan on running, you'll be fine with the 30mm offset. If you go up to 19"s, consider 32F/33R offsets. That's about max before you have to start rolling fenders. Wheel widths and tire sizing will also become more of a factor.

Good luck, and take some pics!

-Christian

Lotus99

Thanks for the lesson! I hadn't considered the poke issue, but had thought that at least in this car, since it's not lowered, with the big wheel gap, running into the fender would be pretty tough, whereas my stock 335 vert (with sport pkg) riding on 18's, with only 12 mm spacers rubs on dips in the road!

Yeah, I can't see 13 mm's more than OEM rubbing or poking out...
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PS I already have the TPMS sensors on my 17's. If I buy a set of 18's (wheels and tires) already mounted from Tire Rack, the local shop would have to take the tires off to put on my old TPMS sensors, right? Or am I just better off buying them unmounted and letting the local shop put on sensors and balance them?

New sensors mounted at Tire Rack will run me $200, so I am going to reuse my old sensors if it makes sense.