dvs_dave

Indeed.
Nicely illustrated by TopGear Oz giving various hot versions of the Holden Commodore (aka Pontiac G8 R.I.P) a workout.
The comparo starts at 6:30 if you don't want to watch the whole piece.

MP4.2+6.0

If it is too harsh for you, back up a step to recall its a combination of wheels+tires, air shock units and roll bars. Various ideas:

1. The wheel size step down you are thinking. But if you do it in factory wheels, try to hold the 9" width to get to the wider tire sizes that give you better braking, etc. Not sure of some of the 19" D3 variants, but the RS4 wheels as an example are 19 x9 and the offsets are such they could work well. Per a mention below, its the pie plates that are the forged and lighter ones, but I don't recall being 9" wide.

2. The 275/35 20's along the lines you are thinking. Simple and likely less $ than even just replacing your stock tires when they wear out. Probably subtle change, but you will find more tire choices available at generally better price points too.

3. Look at your existing speedo error, in part for #2 but more for what I will throw out next. A smartphone GPS/speedo app should give you the data you need just driving down the freeway w/ cruise on, taking some readings and then doing the math. I expect you will find it optimistic, but see. Then if you have more margin for speedo error (or don't mind it being off), and would like the look of filling out the wheel wells a bit more, consider yet another size up: 265/40-20's. What the D4 is now using on most of the cars I've seen. But, not a lot of tire choices, and may not want to go here generally as a "non-D3" size.

4. Consider a wheel change w/in 20" factory choices. Issue is unsprung weight. I think your S8 factory wheels (either of the two primary types depending on early or later years) are cast rather than forged. A guess it they might be 5+ pounds heavier than the factory sport 20s as an example. Buried in the various replies about various of the wheels and tire sizes owners opine about will be underlying construction and weight differences, even factory to factory comparo's let alone aftermarket. If you get a weight on yours, I can give you some of the other D3 factory alternative weights. Total weight of an existing wheel + tire on your car less tire weight per TireRacklisting (if available) can get you an approximation. Like #1, this costs you more $$ since it's both a wheel and tire change.

5. Air unit--not practical; thus skipping.

6. Roll bars. Subtle S8 difference. Your bars are the stiffest "S8 level," which is one up from "Sport" and another up from the non-sport. My W12 came with Sport level bars and I went the other way toward S8 bars which did firm up the ride some--my preference. Another W12 owner (CovertW12) reported the same conclusion. Simplifying it, I have found a bar change is in the same zone as a change from a standard to a heavy duty/firm type conventional shock, but less than a true "sport shock." I haven't done a roll bar change back to back against a wheel size change as an A/B comparison, but it might again be similar. Taken one more step, if you want to affect it only in part, consider a front roll bar change first. Easy to change compared to the rear, and it's the rear bar that would tend to dial in more understeer if you change it (that is, going the other way toward stiffer and more responsive, the back bar is the one that tends to pull out more typical factory dialed in understeer when stiffened). Bar change cost if you have it done (for both) is probably similar to a good set of factory used wheels; front bar would be meaningfully less in labor from having done both, and the front bar is pretty easily in the realm of DIY for anyone with some home mechanic experience.

mishar

For TopGear guys.

But love is still blind.

MP4.2+6.0

Let's stay factory to factory on wheels first. You'll tell me about your 285's perhaps, but its a non stock fitment on a non OE wheel. Most board participants probably have factory wheels so I stay where most of the board OE fitments apply.

In 20's the choices are 9" and 265/275 in tire sizes. In 18's and 19's the choices are typically 8.5's, with some 9" uncommon variants out there. I'm excluding specialty winter OE wheels too of course. Tire sizing is 255 stock in 18's and 19's on D3's. Most any car test I've seen over the years tends to give the normal stopping advantage to the wider tire fitment, and given ABS and plenty sufficient (Autobahn level) rotor sizing, other elements of the brake system for a single stop from speed tend to fall away. Tire selection obviously plays into it too.

In the real world I'll take the x feet shorter stop distance in OE choices so my ride doesn't also have very ugly accident damage if I choose to drive the car with more spirit. Nothing like a smashed front clip to reduce the "fashion" coefficient and hike the premiums.

MP4.2+6.0

I get the general points, but then they need to be adapted to D3 specifics. Below about actual D3 tire sidewall measurements, in other ways for unsprung weight as another example the suspension has to handle (forged vs. cast depending on which D3 factory wheel fits in here).

D3 20's are 275's (265's on S8's). Makes tire diameter 27.7" vs. 27.0 on stock 255's with the 19's. Thus of the nominal 1/2" sidewall delta from 19 to 20," 70% of it is actually dealt with in the specific tire fitment. From 18 to 19" the .5" sidewall delta holds because the tire width is also constant at 255. Said another way, I'm struggling to see how 0.15" in sidewall is meaningful here from 19 to 20" That's within the wear delta of the tire tread itself over its life.

On 18 to 19 increment the sidewall difference has more mathematical validity for the D3. BUT, there the typical 18" five spoke cast OE wheel is meaningfully heavier than the 19" OE pie plate forged, so unsprung weight actually goes down moving from 18 to 19. 19" five spoke may be different; don't know as many specifics there) Net, unsprung weight is a clear contributor to all of suspension wear, wheel bearing wear and overall ride comfort, and due to Audi OE wheel specifics in the 18's, it's not optimized.

mishar

Why only factory wheels? They are harder to find and more expensive.

If we skip unnecessary limitation to factory wheels there are much more choices for 18's. 18x9.5 ET45 ideally accommodate 285/40x18 tires with fine sidewall cushion, big contact surface, well adoptable to a negative camber. If one is willing to pay OEM price (even used) he can buy quite lighter forged aftermarket wheels. Or he can have two sets of brand new quality wheels. One for winters.

Braking wise, let's get back to an imperfect pavement. Every crack and bump will make stiff low profile tires to loose contact a bit, making stop distance longer. A8 brakes may seam to be oversized when braking from 40 mph, but braking from a high speed is a different thing. Two plus tones will fade brakes way before tires begin to slide, so lower moment of inertia can make difference between hard pounding hart and not pounding at all.

MP4.2+6.0

Non factory wheels on Audis and most other cars are typically value subtracts, pretty simply. The average buyer if the car is ever sold looks at it and says, hmmm looks like car may have been abused or owned by a "kid." Yes, one can keep the OE wheels, but then they have to be stored for years, which isn't practical for most. And as a style matter, look at most any 10+ year old set of non OE wheels on an older Merc., Audi, BMW, etc. and my sense is they look more dated and not particularly classy. Yes, there are some quality aftermarket wheels out there, but big picture that's a fringe high priced market and 95% instead is probably the lower quality knock off stuff, and usually cast and heavy. Finally, on this board nothing is suggesting to me that most (or even a lot) of posters are on non-OE wheels, so I try to provide ideas, experience and information that fits the way most folks have their cars actually set up, and to deal with facts specific to those set ups (tire diameter deltas, widths used, etc.)

Thus back to factory, in 18's that means 8.5", which means a 255 tire. Beyond that is non spec. by either Audi or the typical tire manufacturer. As I have posted before, I have also indeed run a set of 18 factory wheels on my W12 for 1000+ miles when my older 20's were refinished. The tires were relatively new, but admittedly the mediocre Pirellis many D3's shipped with. Yet again, probably a common tire on board member cars, maybe even the hidden most common, given so many started that way. Back to back, and whether with summers or A/S's in the 20's, night and day difference. Much higher cornering speeds, much better braking, much crisper feel all around. Yes the 18's maybe were a little plusher, but if I want plush I guess I think Lincoln, Buick or Caddy or some such and less a sleeper Autobahn burner The 18's were borderline dangerous when moving from the 20's down. Some of that is the Pirelli mediocrity, but I have little doubt a fair amount was the basic tire footprint as well as sidewall flex and other things inherent in the OE size delta.

I do agree quality D3 OE's now harder to find. Prime time is a few years into the car's current run where takeoffs are common. That was back when I got the W12 2007'ish and I could by a set of OE takeoffs for hardly more than the tire cost and the TPMS's. Thus now the D4 20's are kind of a dime a dozen in the current model run, and D3 flavors harder to find. Of course I still have the D3 factory 18's gathering dust... Meanwhile, for anyone else reading, my experience is Audizine tends to be a better hunting ground than many of the other Audi boards' classifieds for wheels, OE or otherwise.

mishar

I agree about aftermarket wheels and resale value in general, but that never stopped me to do what I like with my wheels or rest of my car. I don't like saving her for the next owner. Thus back to the free market and real performance value.

On the other hand I do not validate any conclusion about tire sizes drawn by comparing different class of tires. Especially by comparing ones of the best with ones of the worst.

TSHong

The difference between a 35 vs 40 aspect ration is obviously 5%. I think splitting hairs between the two in the real world is not relevant. I'm in the 20's camp. That is why I have 275/35R20 on my car. I can tell you for a fact that 20s cost less, and have more choices than the 19's for any tire line. This holds true in Michelin, Goodyear, and Dunlop. I am an authorized dealer for these tires and I did some research before I put tires and wheels on my D3.

Yes, taller sidewalls have better cushion but you are only talking about a 5% difference. That is just not enough to make a huge difference to offset the cost and difficulty in finding 19 inch tires. The way the industry is going, I bet 19's are going to be hard to find in the next 3 to 5 years. If you can find your size, I bet you will pay at least a 30 to 40% penalty over the 20 inch fitments.

mishar

That's wrong math. 285/40x18 have 18 mm more of a flexible sidewall than 275/35x20. that's 35% more of a cushion. It is not only important for a ride quality. It is crucial for handling, braking, tire wear and security.