Which tire size for softest ride on 8.5Jx18 wheels?

[WeekendDriver]

I'm about to buy a set of 8.5Jx18 wheels (ET 45 but I can do some reasonable spacers if need be) instead of my bone stock 16-inch ones, and I need to get some tires, too. And the thing is, I want the highest possible tire profile for softest ride (i. e. as much air cushion as I can get). And I'm also not sure which tire width fits 8.5J best. I do want the tire to be a tad wider than the rim so that when I park alongside a curb I'll scratch a tire and not a rim.
So my question is two-fold:
1. Which tire width to get for a 8.5J-wide rim?
2. More importantly, what's the highest possible profile that will fit B6 without rubbing on a 8.5Jx18 wheel? Let's assume regular suspension, not the sport one.

Here's what I know. The factory size for R16 is 205/55, and I know for a fact 205/55 R17 also fits without problems. A couple folks run that setup. 205 x 55% = 112.75 mm tire height. But that's R17, and I'm about to run R18. To maintain the same outer wheel diameter, I need the tire profile to be half an inch shorter. So, 112.75 - 25.4/2 = 100.05 mm. So I should aim for 100 mm tire profile height. That would be, for instance, 230/45, or 250/40. Unfortunately, the 5% profile height stride doesn't allow for fine tuning.

Thoughts, advice? Are my calculations sane? They do not account for the fact the my wheel is at least an inch (or even 1.5") wider than the R17 wheels folks run with 205/55...

[JDillon15]

I'm about to buy a set of 8.5Jx18 wheels (ET 45 but I can do some reasonable spacers if need be) instead of my bone stock 16-inch ones, and I need to get some tires, too. And the thing is, I want the highest possible tire profile for softest ride (i. e. as much air cushion as I can get). And I'm also not sure which tire width fits 8.5J best. I do want the tire to be a tad wider than the rim so that when I park alongside a curb I'll scratch a tire and not a rim.
So my question is two-fold:
1. Which tire width to get for a 8.5J-wide rim?
2. More importantly, what's the highest possible profile that will fit B6 without rubbing on a 8.5Jx18 wheel? Let's assume regular suspension, not the sport one.

Here's what I know. The factory size for R16 is 205/55, and I know for a fact 205/55 R17 also fits without problems. A couple folks run that setup. 205 x 55% = 112.75 mm tire height. But that's R17, and I'm about to run R18. To maintain the same outer wheel diameter, I need the tire profile to be half an inch shorter. So, 112.75 - 25.4/2 = 100.05 mm. So I should aim for 100 mm tire profile height. That would be, for instance, 230/45, or 250/40. Unfortunately, the 5% profile height stride doesn't allow for fine tuning.

Thoughts, advice? Are my calculations sane? They do not account for the fact the my wheel is at least an inch (or even 1.5") wider than the R17 wheels folks run with 205/55...

Check out this site : http://www.willtheyfit.com/

It'll help you with your tire sizing.

[WeekendDriver]

We need to account for the specific wheel well dimensions and shape for best result, but thanks, neat site! I like how it visualizes ET. But, again, it's all just a game of numbers without taking note of the specific car.
I mean, if I were to input my stock 205/55 R16 dimensions, I would never know that 205/55 R17 would also fit perfectly.

[demonmk2]

40 or 45 *235* series tire if you want smoothness/comfort.

you could probably go to a 50 or 55 but.....it would not look good at all.

[MacFady]

If you are worried about curbing I would recommend buying a tire that has a small rubber lip as a feature specifically for this purpose as opposed to buying a tire that is a "tad wider" than the wheel. Otherwise just measure how much space you have to play with and play around with any of a number of different online resources for tire sizing such as 1010tire.com. I would guess 235/45 will fit without issue, 235 is a good middle of the road size for a 8.5 wheel, 45 sidewall is only a little more than stock and likely your best option for a little more comfort without looking funny.

[WeekendDriver]

you could probably go to a 50 or 55 but.....it would not look good at all.

That's not a concern. If anything, I like the brutal look of big fat rubber you can use to traverse some mud with fearlessly :)
I'm a bit skeptical as to whether 235/45 would fit rubbing-free in the front (and even more so about 50), but if it would, that would be awesome! Almost no sacrifice relative to my current setup.

If you are worried about curbing I would recommend buying a tire that has a small rubber lip as a feature specifically for this purpose

I've heard about those, but never seen such a tire myself (I think). What's the name for that, how can I search for it?
Anyway, I will pick tires by performance stats upon reading some tests, but will definitely keep an eye out for that rubber lip.

Otherwise just measure how much space you have to play with and play around with any of a number of different online resources for tire sizing such as 1010tire.com.

I have no idea how to measure inner clearance, other than maybe when the car is lifted (but I have nowhere to lift it without paying big bucks).
Thanks for the 235/45 tip. At the very least now I know which width to aim for.

[MacFady]

I have no idea how to measure inner clearance, other than maybe when the car is lifted

I was just referring to when using online calculators for the tire size you're looking for compared to what you have. Input what you have for a tire/wheel size, take a tape measure when the car is at ride height and see how many inches/cm's whatever you have to various components such as your fender in the rear. Up front it is helpful to measure to the suspension components when the wheel is turned to full lock. Now that you have a ballpark idea for clearance you will know what you have to play with when the new values are displayed compared to what you currently have when using something like the 1010tire.com calculator.

Also, your ET is 45, so if you find a tire size you like but it looks like you are going to be hitting the suspension up front when turned to full lock you can consider what spacers might move you away from that suspension while at the same time not hitting the fender. I'm just trying to give you something to work with here, but 235/45/18 will be fine even with sport suspension. Work with 235 as your width in the calculator and just see what you can get away with for the sidewall.

[WeekendDriver]

Thanks a lot!
I've also been told (by a person I trust because he knows A LOT about our cars) that there was a B6 version that came stock with BBS 8.5Jx19 ET 35 wheels - I just don't know which profile (obviously a slim one). And that car had the sport suspension. So at the very least a 10 mm spacer would solve some problems for me (ET 45 -> 35). But I still need to be reasonably sure that the tire height I pick can be fitted, else I'll be losing about $500 USD, you know (going to get a new set of tires, not used ones).

[CyberPMG]

The standard tire size for an 18" wheel is 235/40/18 which was what came on the Ultrasport wheels (18x8) which has the Sport suspension. You'll have a tiny bit of stretch going to a 8.5" wide wheel which should give you more suspension clearance.

[WeekendDriver]

Aha! So 235/45 should fit no problem, but it's not the best width for 8.5"?
235 mm + 1/2" = 250 mm. That's one wide expensive tire...

[CyberPMG]

Keep in mind that with the standard non-Sport suspension, the car will sit higher, but with softer springs and struts, there will be more wheel travel. The key is to make sure you're not going to interfere with the control arms.

All of these tire sizes maintain the same outer diameter and are the standard sizes that came on the B6 A4 regardless of suspension:

15" - 205/65/15
16" - 215/55/16
17" - 235/45/17
18" - 235/40/18

Going to a taller sidewall will increase the overall diameter of the tire and throw off the speedometer. I think you're one of the first to ask to go for a taller sidewall, so there's not a lot of others who can chime in here. If you have a friend who can let you try a set, that would be the best way to know for sure.

[WeekendDriver]

If you have a friend who can let you try a set, that would be the best way to know for sure.

I understand that's the only certain way, but sadly, I don't have such a friend.
I've seen only one person specifically asking for a taller tire (on a different forum), but I've seen at least 4 people saying "screw it, I'm selling these R18 / R17 and going back to stock 16" because the ride is too stiff". I want a new shiny polished set of wheels, and I don't want to be one of those who went back!

And yeah, I understand speedo / odo implications. Is that not correctable via VCDS, by any chance?

[CyberPMG]

You can use VCDS to change the tire size to have the odometer adjusted, but you can only choose from a preset list of tire/wheel sizes (the list I've mentioned above).

[WeekendDriver]

Thanks a lot for the info. But now I'm at a loss: you said all those aforementioned tire sizes amount to the same outer diameter, but in that case the speedo / odo readings would actually not be affected by swapping tires to a different size form that list. What am I missing?
P. S. I just ran the math to compare 215/55 R16 to 235/40 R18, and indeed, there's only ~2 mm difference.