Oh no, TIRES again! R888R's or Cup 2 R's
 

I know tires have been covered a zillion times here. But, this is more about size and stock. So basically WANTED crazy wide rears, but stock size fronts. I never drive my ZL1 in bad weather (not even much in cold weather) and never in wet conditions. I was looking for the widest rear possible for my 2023 ZL1 (non-1LE) that will grab the pavement the best, and with out other mods required (other than maybe a 3mm or 5mm spacer disc for the rears).

I wanted the Toyo R888R's in 345/30. But, seems like they only make the 345/30 in R19, not R20 like I need. So now I'm looking at the Michelin Pilot Sport Cup 2 R's. Rears I'm going with 345/30R20 and the fronts I planned on stay with the stock size, 285/30R20. I wanted to know: Is mixing tires of like this ok and won't cause handling issues. Also, I'll never track my car (road course) and only straight line stuff. I'll make sure to check the clearance on the driver's rear for the inner line clearance. But, other than that, that's my plan. Thanks for any input!

You are about to go from 305/30/20 to 345/30/20, which increases tire diameter/circumference by 3.3% while leaving the fronts stock.

In many cases people saw Stabilitrak, traction control and/or ABS weirdness when exceeding a ~3% margin of difference between front and rear with these size changes, so I would not recommend this plan. To be on the safe side, either increase the height in the front as well, or go a smidge narrower in the rear.

Of course if you're hardcore and keep the nannies off all the time, you may chance it, but I don't think that's a good idea even then... taller tires can improve max speed but are worse otherwise, and your rears would be at ~28.1", almost an inch taller than stock.


EDIT: one more thing, those Toyos whine like a monster truck (been there, heard that, sold them), so unless your setup is super loud in the cabin, it may well have bothered you, so don't feel bad you had to forgo them :D

If you're going that territory... look up Nankang CR-S... the 911 hot-shoes swear by them. Those guys know a thing or two about sticky tires.

Better than both of those.

Good info! OK, this is why I post my "dumb" questions here. I know I'll get answers from those who know their SH!! I don't want a taller tire, never thought of how the tire numbers worked together like that! I didn't want to change tire size in my computer, although I have HPTuners so I could. I would just rather not. Thus, maybe should go with 325/xxR20, and try to keep same over all height somehow? Wider in the front I'm not a opposed to. I just didn't need to change them to "change them"! Thanks again for the info!!!!

went with Apex 19s and 325/30 r888rs in the back. Stand by till the snow clears off the roads here and they ground temps warm up.

I am confused, ZL1-Auto is still sticking with the XXX/30/20 ratio am I missing something here with the diameter? Is it slightly different with tire manufacturers?

A 305/30/20 has a 3.6” sidewall and a 27.2” diameter across the tire. A 345/30/20 has a 4.1” sidewall and a 28.1” diameter. Tire manufacturers play a little role, but a 345/30 means that 30% of the width is the sidewall, so a wider tire with the same aspect ratio means a taller sidewall, and taller overall tire. Hope that helps. :)

Thank you, I get it now. So my 285s and my 305s fall under the acceptable percentage difference.

As Josh said, the second number is a percentage, not an absolute value (don't even get us started about why the first number is in millimeters, the second one is a % and the third one is in inches :smiling1:).

Let me clarify the "difference" part a bit. The OEM tune in the car assumes 285/30/20 in the front and 305/30/20 in the rear, this is what the circumferences (in reality, wheel speed sensor calibrations) in the tune are set to, and this is the configuration Stabilitrak, traction control and ABS are programmed to.

Now, this means the car assumes a specific ratio between front and rear wheel speeds (circumferences/tire heights, all the same thing practically). As long as you don't deviate from this ratio too far, you'll be okay. Front and rear changes have to be considered together---say, if you increase the front tire height by 2% and rear tire height by 3.5%, you will only be 1.5% off the "expectations" of the car, and it has enough margin built into the logic to tolerate this overall difference up to ~3%.

Another tricky situation is when someone reduces tire height on one end and increases it on the other, because the difference will be the sum of absolute values then (e.g. -1.4% rear and +2.1% front is unsafe, because the aggregate difference will be 3.5%).

So, when OP wants to change the front by 0.0% and the rear by +3.3%, he is about to enter the risk zone.

I wish I had access to GM's source code, but I don't, so we have to rely on conjectures and trial-and-error for our conclusions... however, in this case there have been many instances of people plagued by problems after drastic tire size changes.

There are a couple issues with what you are wanting to do:

-Increasing only the rear tire size can negatively impact the driving dynamics of the car.

-A 345 on an 11 inch wide wheel is way too big for that width.

Also a good track tire doesn't necessarily make a good grippy street tire. A Cup 2 R is a qualifying tire. Meaning its meant to performance one or two extremely fast laps and then the top sticky layer comes right off. Car manufacturers use this tire to claim lap times/records.

Also, rack tires aren't made for instant grip. A Michelin Pilot Sport 4S is going to provide more grip and traction on the street than a Cup 2 or Cup 2 R.

The R888R does actually provide instant grip on the street. It meant as a track tire, but its not that great of a track tire. It makes a better street tire.

If I were you I would lose the idea of putting the widest rear tire and either go with the R888R in OEM sizes or do the PS4S in 295/30/20 front and 315/30/20. You should have plenty of grip.

Skip the spacers. You are not going gain anything from them especially if you only want to do the rears.

The Nankang CR-S V2 is an excellent track tire, but its not going to provide the instant grip and track of a PS4S or R888R that the OP is looking for.

Thanks everyone! That's all good and in-depth info. I should have mentioned originally that my car (the tires I put on the car) will NEVER be on a track (lap tracking). I might, only ever, go to my local drag strip (straight line). I understand now that a big changes can cause drivability issues and I'll make sure to keep this in mind. I did find this that helped me understand this visually!

https://tiresize.com/height-calculator/

For instance:

305/30R20 = 27.2 diameter & Revs/Mile : 742 @ 65 mph
345/30R19 = 27.1 diameter & Revs/Mile : 743 @ 64.87 mph

I am leaning toward the R888R's or the NT555R2's. Sounds like I will be sticking to stock size. But, I would like a bit wider in the back. Seems like I would have more options if I went to 19" wheel vs keeping my stock wheels. I am going to go work with my local tire guy Nick at Discount Tire (the happen to own Tire Rack too) and will get this figured out! Thanks!

Just for reference, I took a screenshot of wheel and tire size for the 2023 ZL1 and 1Le from a GM web page:

OP, are you road racing? Or just aggressive street?

I ask because I don't track mine and I love the grip I get from the Nitto NT555RII in 315/35R20.

Like you, I don't drive my ZL1 in wet or when it is below 45 degrees F. And these Nitto's really launch great from a start.

He's got a ton of modded power that he's trying to get to hook up but you can only go a little bit up or down in sizing, it's better tire that is needed not necessarily bigger (wider). The difference as most have said, front to rear, is going to make the TC/ABS, all that, go banana's because its outside the tolerances that have been set in those systems. I think you're on the right tire the Toyo, but have little room to play with sizes. 315/30 or 35 is about it...