Technically speaking, can I run 255/35 19" in front and 235/40 19" in the rear? If I recall correctly, that can damage the haldex systems but does not affect the torsen Quattro systems as in the B8. The front would be 26.0" tall and the rear 26.4" or so. I already run a 10mm wider front track with 5mm spacers on either side.
Why would you run different sized tires ? But I believe Audi says to keep the tires within 3% of each other.
I was always told to run the same size tires all the way around on Audis unless it is an R8. We had some stanced cars that ran different sized tires and they broke the center differential.
Why can the r8 run different sizes with no issue?
The new RS3 actually runs 25.0" front and 25.5" rear so modern haldex may be more forgiving.
U will be working the center diff more, and it will screw up abs amd stability control due to different velocity read outs per axle. I wouldnt do it. Unless the car was configured for it, you are messing with the driveline too much imo. It may even affect the behavior of the active diff due to dofferent axle velocoties front and rear
Edit: boro gave a pretty quick and easy explanation of what I was writing below at the same time…
It doesn’t have anything to do with the type of AWD system used whether it’s Haldex or Quattro. Both of those systems use some sort of clutches in the center differential that can be problematic if not setup or tuned for different sized tires front and rear (or side to side). Let me see if I can explain this…
In terms of the basics, Quattro systems use a center (limited slip) differential with clutches that lockup. It’s using a Torsen differential (B8 S4) or Crown Gear differential (RS5/B8.5). These types of systems work better on a RWD biased system. The Torsen system is 40/60 torque split by default with up to 80% to either the front or rear depending on the amount of slip detected, and the Crown Gear can send 70% of torque to the front or as much as 85% to the rear.
The Quattro center differential uses clutches that are compressed (or uncompressed) as “slip” (the speed difference on each axel/side) is detected to determine the amount of lockup on each side. Running different sized front and rear tires replicates a slip/difference in those two sides equal to the difference in tire sizes. So, if you have a 3% difference in tire sizes, the system is starting with 3% slip off the bat. If the differential is setup/tuned to start lockup at 5%, it’s obviously a lot easier to cause it to lockup at that point. If you run a big enough difference (over 5% in this example) and it’s constantly locked up, that’s where you can cause (fast) premature differential failure. Differentials are obviously not designed to be locked up all the time. It would create a lot of heat and unintended consequences. A 3% difference on a system designed to lockup at 5% probably won’t cause short-term failure, but could have some long-term impact from more frequent use. More importantly, it will effect the handling dynamics of the vehicle; especially if it is driven at or near the limit.
A lot of people will say Audi’s can have XYZ(%) amount of tire size difference, but the fact is, no one really knows exactly how Audi sets up their various AWD systems. Every vehicle is slightly different in the way it is setup or tuned. The general consensus is that the differential will start to lockup when it detects something like 5% slip, but it’s a lot more complicated than that (the center and rear differentials on Quattro are also using sensors to detect yaw, steering angle, etc. as inputs to determine lockup). That leads to the idea that something under 3% is OK as it leaves another 2% before it starts to lockup. In reality, if I was driving a car at the limit, I wouldn’t want to start with any difference in tire sizes. For daily driving a 1-2% difference probably won’t matter in the short-term, but 2-3% will absolutely adversely effect the handling dynamics as you get closer to the limits.
The Haldex system uses a hydraulic pump with the two sides connected via wet clutch packs. As slip is detected, the pump starts to pump more oil causing the clutches to lockup more. This type of system is better at sending more power to the front wheels, or something close to a 50/50 split. At the end of the day, it’s still using clutches, and can be setup or tuned however Audi wants in terms of lockup. In the case of the RS3 with different tire sizes, it obviously setup for that through the use of different clutches and ECU tuning.
Both systems can be tuned or setup to lockup however Audi want. This is done through ECU tuning of the sensors used to control the differentials, and also the number/size of the clutch packs in the differentials. Different clutches can be used to cause lockup sooner or later or whatever.
Great responses and I’m learning a lot. Regarding the RS3, we’re not sure if there is dynamism or programming. Meaning the sport tire setup and the standard setup may have the same program and allowable differences. Audi has electronic brake force distribution so you can vary the front and rear pad friction without locking up either side. As I understand it the sport diff responds to yaw and steering angle rather than wheel spin.
I honestly don’t know the exact specifics about how any of the systems operate. Conceptually, I have a pretty good idea as to what they are capable of, but it would take an Audi engineer or someone a lot smarter than me (not hard) to really figure out how it’s executed on a particular vehicle.
I will add that in general I’m in agreement with boro. It’s not something I’d typically recommend for the reasons he listed. If someone was dead set on it, I’d always recommend the smallest difference possible (usually this comes up when people are asking about staggered setups).
Modern AWD systems like Quattro are very finely tuned both electronically and mechanically, and adding in a significant 2-3% difference in tire sizes just seems like a lot. If you could take the electronics out of it, it’d be a lot easier to know exactly what’s happening from a mechanical standpoint, but all the other sensors and tuning involved leave a lot of unknowns.
A note there is NO HALDEX on any Longitudinally mounted engine car from Audi. The AWD Drive system is mechanically engaged full time AWD with no control unit. The Sport Diff option on the S Cars has no tune or adjustment exsists that I know of since it works off steering angle, abs, and throttle position sensors for the most part in an attempt to limit understeer when on the throttle.
We’re talking about 787 revolutions per mile vs. 799 revolutions per mile, so a 1.5% difference. That’s the sort of difference that would be lost if we were talking about a car that made mostly left turns around a counter clockwise road course. I think.
The real question is - why run different tire sizes.
I suspect you are after a change in handling balance–which can be addressed with all the adjustable components you got on your car. No need to compromise with barely rotateable tire sizes.
An OE car with staggered sizes has to constrain to an OE alignment (read: barely any camber, and 0 toe at best). The only solution there is to throw a bigger tire at whichever axle needs more grip. But now that you have adjustable bars, adjustable upper arms, adjustable corner heights and adjustable damping, there is little reason to buy tires to address a handling trait when you have so many more optimal options at your disposal. You made the purchase for top shelf adjustable components–adjust them 
IIRC you are running the shocks very soft. Play with them front/rear and get the car to drive how you like.
When you are running 2 different size tires, you will have 1 tire heat cycle out before any other, and with the lack of ability to do a real rotation, you worsen this problem and you will find a performance drop off in your tires sooner than you normally encounter them.
FWIW I already found that my Dunlop Z2’s were good for 2 heat cycles (well, 2 track days), and performance dropped off. I cannot imagine having a tire which I cannot move from front to back.
+999 for Jran’s detailed post. Bang on.
I’ve greatly changed the weight balance (59% front) and purpose of the car from factory. All of the weight reduction has come off of the rear end of the car and with no back seats, it will never be “loaded” again with passengers or gear. A 255 XL is overkill for what I’m asking the rear axle to do. A 235 XL cup tire will provide me with plenty of traction and reduce drag. It will also tuck under the fender better - right now a 255 can push on the bumper cover where it meets the wheel arch under full cornering load.
I’d like to run a 19x9.0 forged 16lb wheel in front and a 19x8.5 flow formed 24lb in the rear. The fronts are doing a ton of work still.
I don’t care about rotation and never have. Tires only need to last 10 track days and 3000 miles.
Another option:
Pirelli Trofeo R
front: 265/30ZR19 (93Y) XL @ 25.3"
rear: 235/35ZR19 (91Y) XL @ 25.5"
FWIW the weight of the rear axle has nothing to do with tread width.
What treadwidth is right for you is a factor of tire temps and the amount of lateral grip you can generate.
If you look under prepped time attack cars, you’ll find sub 3000lb 2wd cars running 255 tires.
The other thing you need to take into the equasion is rim width. The width of the wheel will affect the stretch of the sidewall and ultimately dictate the true rolling diameter of your chosen wheel/tire combo.
Crazy though. My tires last me 4 days – with rotation and flipping. How do you get 10 @_________@
After 4 days they are very very heat cycled. and my tread starts splicing after 2 days 
You might be rolling up to the grid over 500 pounds heavier than me with a right seat anchor.
Holy cow. Fair enough. Your car weighs in at 3500? Fwiw i dont normally take passengers bc i am all for laptime lol. So it’s just me in the car. With me it is probably just over 4k lbs i think
3600 empty, 3800 grid weight.
Related but unrelated. I thought this was an interesting comment in an article about the new Focus RS AWD system:
Initially they tried a Haldex unit, which provided great straight-line traction but tends to be dull at the limit and usually results in understeer. Ford said the team tried fitting larger rear tires to mechanically overdrive the rear wheels and counteract the limitations of the Haldex, but the driveline blew itself to pieces due to the different front and rear ratios and no real way to slip one of the ratios to even everything out.