I've been searching the forums and there isn't a definitive answer about whether you should go 285/30/19 or 285/35/19 as there are people running both sizes and was wanting feedback. I currently have 255/35/19 on the fronts as well. Any thoughts would be appreciated. Thanks! (BTW I have a sedan with the 19 inch wheels)

You use a tire comparison tool and see how they compare. The main difference will be the diameter and that will throw off your speedometer. Your 255/35 is already smaller in diameter compared to the 245/40 OE size. The other thing to keep in mind is wheel width. Don't put a wider tire on too narrow of a wheel. You also don't really wanna go that wide in the front. You'll likely just get a ton of tramlining, and you need to have sufficient fender clearance to not rub under any kind of suspension compression.

https://tiresize.com/comparison/

https://www.tiresandco.ca/tire-equivalence-advice.html

OE tire size is 245/35/19 in front for the sedan.

Must be a typo in the 2019 DOG I have. It says 40 aspect ratio.

I tried the tire comparison tool and the 285/30 was a -2.1 at 90mph while the 285/35 was a +2.1. I see a lot of people running both sizes so I was trying to see if there is a general recommended. I currently have a 295/30/19 on right now and lowered but found I slightly understeer in hard corners so was looking to go to a 285.

The general recommendation is equal or smaller diameter than OE, so that your speedometer never shows less than actual speed. Also keep in mind that if the revs/mile front and rear are off by too much from each other then your ESP may get confused and not work correctly, and in the 2019+ FL models you may confuse AMG DYNAMICS which does torque vectoring and actively shapes the handling of the car. You didn't say what year your sedan is.

I have a 2020 but I currently have 295/30/19 with no issues, so just trying to go to a 285.

I just mentioned it because you said you are understeering. That's something AMG DYNAMICS is supposed to eliminate. FWIW, the 63S coupe stock size is 255 in the front and 285 in the rear and it doesn't understeer. 295 isn't that much more, so your issue is likely the modified suspension and not the tires.

I have maybe assumed it may be the springs, but visually compared to the 295, the 285 bulges much less and looks like it fits nicer. Also, if traction control is off, does Amg dynamics still affect the car? I would assume so but just wanted to check.

Yes, also consider that the OE setup slightly stretches the tire to keep it from rolling over the sidewall during hard cornering. The wider you go w/o also changing out the wheel for a wider wheel, your setup will be more square and it will roll over the sidewall under hard cornering. Most people go wider for off the line grip (more rubber), but it will actually negatively affect the handling because of the more square setup. Below is a pic of my coupe with the 285 PS4S. You can see the slight stretch. The stretch was actually even more pronounced with the stock PSS. The PS4S is slight wider than the PSS in the same size.

Yes, AMG DYNAMICS is always active. It controls the electronic locking differential and still does torque vectoring. That isn't affected by whether traction control is on or off.

Just wanted to re-emphasize, at the end of the day you changed your suspension geometry by lowering it, so that is your much bigger issue. Most off the shelve suspension mods make cars worse, IMHO. I still remember one time I came up on a guy in an Infinity with lowered suspension on one of my canyon drives. He tried to keep pace, but his car was skipping all over. The suspension totally ruined it. He almost flew off the road trying to stay ahead of me while I was casually following him. Your tires likely don't have an optimal contact patch anymore or don't make proper contact when the road gets bumpy.

I'm currently running 255/35/19 front & 275/35/19 rear on my '19 sedan. I like that setup much better than the 245/35/19 front & 265/34/19 stock setup as I get less front end push & better rotation in the rear. Manufactures always lean towards understeer as its much easier to control.

Typically you want to keep the rolling diameter no greater that 3% between the front and rear to keep the ESP/ASC systems working correctly.