[bandier]

I have a 2021 LT1. I finally got my aftermarket 20x10 & 20x11 rims and prepping to get my tires. Stock tires for the LT1 is 245/40R20. I'll be moving up to 285/35R20 & 305/35R20.

I'm aware the speedometer will be affected. Should I be concerned about the CEL going off?

[bandier]

I thought I'd get a response on this. I take it this is a lame question. I don't think I'll have a problem, but I could have sworn I saw some post about someone getting a CEL after changing tire size. Anyone who has beneficial insight, please confirm or deny. Thx.

[ctrlz]

The car wants any changes to tire diameter to be within 3% of the stock diameter. otherwise you get a stabilitrak warning.
Just google "camaro6 stabilitrak tire change" and you will find multiple threads. There are some work arounds if you have a tool that lets you get into the settings.
here's one thread:
https://www.camaro6.com/forums/showthread.php?t=619578

EDIT: sorry, it may be front/rear ratio, not comparison to original tire size.

[bandier]

Thank you Ctrlz. This is the stuff I was referring to. So, I'm installing near 1LE spec tires I should be OK. Right?

Originally Posted by ctrlz

The car wants any changes to tire diameter to be within 3% of the stock diameter. otherwise you get a stabilitrak warning.
Just google "camaro6 stabilitrak tire change" and you will find multiple threads. There are some work arounds if you have a tool that lets you get into the settings.
here's one thread:
https://www.camaro6.com/forums/showthread.php?t=619578

EDIT: sorry, it may be front/rear ratio, not comparison to original tire size.

[bandier]

After reading the thread you shared, I'm concerned. I thought upgrading to near 1LE rims and tire sizes would just affect the speedometer. That thread makes it sound like there will be problems with the ABS and Stabilitrak and there is no fix. Anyone know of a solution yet? I bought the rims and now I'm thinking I screwed myself by not researching the mod enough. Are people upgrading their LT1s just dealing with CELs ??? Please reassure me I can buy my tires and be free of ABS and stabilitrak issues.

Originally Posted by ctrlz

The car wants any changes to tire diameter to be within 3% of the stock diameter. otherwise you get a stabilitrak warning.
Just google "camaro6 stabilitrak tire change" and you will find multiple threads. There are some work arounds if you have a tool that lets you get into the settings.
here's one thread:
https://www.camaro6.com/forums/showthread.php?t=619578

EDIT: sorry, it may be front/rear ratio, not comparison to original tire size.

[WhiteMale]

You can use this to see the size difference, like was said you stay in the 3% and it should be fine. https://tiresize.com/calculator/

Though I think a 285/35 will not fit up front, without a spacer. 285/30 = yes. Also 305/30/20 is what you want on the rear.

[bandier]

Ok. For clarification, I must make sure the front to back size difference ratio is within 3%. Another post mentioned 3% from stock tires size which I’d be in trouble then.

Regarding the 285/35, it seems others are running this and I didn’t see mention of a spacer. I don’t have a problem with a 285/30 but was shooting for a bit more cushion for the occasional pothole.

Others please chime in to guide me before I purchase the tires. Thanks.

Originally Posted by WhiteMale

You can use this to see the size difference, like was said you stay in the 3% and it should be fine. https://tiresize.com/calculator/

Though I think a 285/35 will not fit up front, without a spacer. 285/30 = yes. Also 305/30/20 is what you want on the rear.

[arpad_m]

It's not the individual sizes or the front:rear ratio itself but the delta between the stock front:rear ratio and the new one you're trying to use that you need to keep within 3%. That means you can, say, change the front size by more than 3% as long as you change the rear also in the same direction. As advised above, the calculator at tiresize.com is very useful for determining what configurations are acceptable.

Check out this earlier post of mine for a few examples: https://www.camaro6.com/forums/showt...php?p=11450914. There are many things to consider, your car doesn't have the 1LE tune, so I'm not sure using those sizes is the absolute best.

As to your specific example of 285/35/20 and 305/35/20, I had this setup for years and it will work without ABS and Stabilitrak issues, but you may see some rubbing in the front at full left steering lock on the driver side, because the tire will run a bit too close to the wheel well liner and there is a metal body element behind the liner, so it has nowhere to flex to. The rubbing isn't very bad (unless you track the car, heh), though, depending on the tire you choose, it might be okay. For example, the Michelin PS4S I run is on the smaller side physically even for the same nominal sizing, so if you run Continentals or another brand, the problem may be worse.

Sizewise you will be okay in the rear, but I'd pull back the wheel well liner on the driver side and make sure the main body harness does not follow the original routing that GM used from 2016 but supposedly revised later. You don't want to rub through the liner and damage this harness, because this harness is one of the first parts installed on the assembly line, almost the entire interior needs to come out to replace it. Here is a thread where you can learn more about what to do here: https://www.camaro6.com/forums/showthread.php?t=485975.

Final note, all of the above assumes you don't want to tune the car, because tire sizes can be changed in the tune, which should eliminate ABS and Stabilitrak issues. Even then, though, you don't want to change the setup too much, because the suspension obviously won't change and you can easily make the ride and cornering worse and worse.

As it is with almost everything, wheel and tire selection is a tradeoff.

[carguy55]

I run various sizes of 18", 19" and 20" tires. Just don't mix them. Never had a CEL. Only issue is the speedo is a little off with size other than 20".

[bandier]

This exactly the info I was looking for. Thank you Arpad. I always appreciate your knowledgeable posts.

The one thing I didn't mention is offset of the new rims. The front rims are 20x10 +25. I take this will increase rubbing at full steering lock. The rears are 20x11 +35 which I think would decrease any wheel liner rubbing. Any insight? Thanks again.

*

Originally Posted by arpad_m

It's not the individual sizes or the front:rear ratio itself but the delta between the stock front:rear ratio and the new one you're trying to use that you need to keep within 3%. That means you can, say, change the front size by more than 3% as long as you change the rear also in the same direction. As advised above, the calculator at tiresize.com is very useful for determining what configurations are acceptable.

Check out this earlier post of mine for a few examples: https://www.camaro6.com/forums/showt...php?p=11450914. There are many things to consider, your car doesn't have the 1LE tune, so I'm not sure using those sizes is the absolute best.

As to your specific example of 285/35/20 and 305/35/20, I had this setup for years and it will work without ABS and Stabilitrak issues, but you may see some rubbing in the front at full left steering lock on the driver side, because the tire will run a bit too close to the wheel well liner and there is a metal body element behind the liner, so it has nowhere to flex to. The rubbing isn't very bad (unless you track the car, heh), though, depending on the tire you choose, it might be okay. For example, the Michelin PS4S I run is on the smaller side physically even for the same nominal sizing, so if you run Continentals or another brand, the problem may be worse.

Sizewise you will be okay in the rear, but I'd pull back the wheel well liner on the driver side and make sure the main body harness does not follow the original routing that GM used from 2016 but supposedly revised later. You don't want to rub through the liner and damage this harness, because this harness is one of the first parts installed on the assembly line, almost the entire interior needs to come out to replace it. Here is a thread where you can learn more about what to do here: https://www.camaro6.com/forums/showthread.php?t=485975.

Final note, all of the above assumes you don't want to tune the car, because tire sizes can be changed in the tune, which should eliminate ABS and Stabilitrak issues. Even then, though, you don't want to change the setup too much, because the suspension obviously won't change and you can easily make the ride and cornering worse and worse.

As it is with almost everything, wheel and tire selection is a tradeoff.

[bandier]

Good to know. I was really asking about stabilitrak and ABS warning/errors, not CEL but when I realized my error the forum doesn't allow changes to the post title. I am learning I shouldn't get any warnings with the tire changes I'm doing. Keeping my fingers crossed. The speedometer difference I'm totally cool with.

Originally Posted by carguy55

I run various sizes of 18", 19" and 20" tires. Just don't mix them. Never had a CEL. Only issue is the speedo is a little off with size other than 20".

[bandier]

Arpad, now reading your supplemental post regarding deviation percentages from stock tire sizes. If I did the calculations right... With 285/35R20 & 305/35R20 my total deviation is 3.2% as I have a LT1 and the stock tire sizes are 245/40R20 all the way around, not staggered like the SS. So I'll be in trouble... Correct???

Crap this sucks... I'm going back and forth whether I can do this or not. Without a tune anyway. I really don't want to pay for a tune.

I redid the calculations for 285/35R20 & 305/30R20 and it bring the percentage to 2.5% but then you mention rake changes. Any suggestions for me??? Get a tune I presume.

Let me know if I'm doing this right.

[bandier]

Arpad, If did this right I think I'm screwed without a tune, which I would prefer not to do.

Here's my calculations:
The LT1 has 245/40R20s all the way around (not staggered), thus increasing my deviation percentage drastically:
285/35R20 = 0.7% & 305/35R20 = 2.5% for total deviation of 3.2% (OVER)
285/35R20 = 0.7% & 305/30R20 = -1.8% for total deviation of 2.5% (CLOSE but a rake change)
then your suggested set up
275/35R20 = -0.4% & 315/30R20 = -1.08% for total deviation of .68% (Good but still a rake change)

Did I get all this right? How drastic is the rake change? I really wanted aftermarket rims and I'm starting to regret my lack of research. Suggestions? I guess, get a tune.

[arpad_m]

Originally Posted by bandier

This exactly the info I was looking for. Thank you Arpad. I always appreciate your knowledgeable posts.

The one thing I didn't mention is offset of the new rims. The front rims are 20x10 +25. I take this will increase rubbing at full steering lock. The rears are 20x11 +35 which I think would decrease any wheel liner rubbing. Any insight? Thanks again.
*

You're welcome, bandier. The stock offset on the 20x10 1LE fronts is +20, so you're 5mm off, but you can use 3 or 5mm spacers to adjust that back, so I don't think you'll have an issue.

I'm obviously just a guy and not a wheel/tire/suspension tester, so I can't be 100% sure, if only because my car has magnetic ride control shocks and yours has the base suspension, but I did try many combinations over the years, and the spacers will give you some leeway no matter what.

Originally Posted by bandier

Arpad, now reading your supplemental post regarding deviation percentages from stock tire sizes. If I did the calculations right... With 285/35R20 & 305/35R20 my total deviation is 3.2% as I have a LT1 and the stock tire sizes are 245/40R20 all the way around, not staggered like the SS. So I'll be in trouble... Correct???

Crap this sucks... I'm going back and forth whether I can do this or not. Without a tune anyway. I really don't want to pay for a tune.

I redid the calculations for 285/35R20 & 305/30R20 and it bring the percentage to 2.5% but then you mention rake changes. Any suggestions for me??? Get a tune I presume.

Let me know if I'm doing this right.

Relax, this is nowhere near as bad as you think.

Your new 285/35/20 fronts will be +0.7% (ie taller), your 305/35/20 rears are +2.5% (also taller) compared to the stock 245/40/20 overall height, so you're changing the ratio much less as you're increasing the height on both ends. As you quoted, my napkin calculation here is simply delta = rear - front using the proper sign, so your ratio change is only ~1.8%, which will be just fine.

Your mistake is in thinking these quantities are always additive, but they are actually directional, so to get the actual diff you need to subtract. If you inverted the directions between front and rear (say, taller fronts, shorter rears or vice versa), then you'd have to add their absolute values (magnitudes), but that's not what you are planning. Hope this makes sense.

Originally Posted by bandier

Arpad, If did this right I think I'm screwed without a tune, which I would prefer not to do.

Here's my calculations:
The LT1 has 245/40R20s all the way around (not staggered), thus increasing my deviation percentage drastically:
285/35R20 = 0.7% & 305/35R20 = 2.5% for total deviation of 3.2% (OVER)
285/35R20 = 0.7% & 305/30R20 = -1.8% for total deviation of 2.5% (CLOSE but a rake change)
then your suggested set up
275/35R20 = -0.4% & 315/30R20 = -1.08% for total deviation of .68% (Good but still a rake change)

Did I get all this right? How drastic is the rake change? I really wanted aftermarket rims and I'm starting to regret my lack of research. Suggestions? I guess, get a tune.

The rake change is not super significant, but you would probably feel it as increased lightness in the front when pushing the car, which you don't want.

My choice for a non-1LE like yours and mine would be 275/35/20 and 315/30/20, this will work fine with zero issues, no tuning required. This is the setup I actually purchased last year when the time came to get new tires, and it's just perfect. The only problem is the limited availability of this infrequently used rear size, but as I said, everything's a tradeoff