I just had 4 new Michelin Pilot Sport AS Run Flats put on my 2018 Z51. I checked the tire pressure today and noticed 305/30ZR20s were installed in the back instead of the original 285/30ZR20s. Has anyone had any experience with this combo? Do you think there any potential issues with this size instead of the original?

 

According to this calculator there is a 1.9% difference in the circumference. Probably not enough to make a difference but traction control uses wheel speeds as one of the parameters.

 

The fronts are still 245s? Welcome to understeer. That will throw your handling balance off.

 

Did you authorize this substitution? I would ensure the correct size for safety that the stability control works properly and especially since you have an E diff. The sub might have been made due to availability but it's not acceptable especially on a car like this.

 

Thanks for the responses. The fronts are 245s and I did not authorize the substitution. I'm waiting for the installer to call me back regarding replacement.

 

Me too

 

The shop should not have put wider tires on the car without notifying you and getting your permission to do so. They should make the change at no cost to you if you do not want the 305s on the back.

 

Keep us updated on what happens here, good luck with getting the proper tire installed !
Rich

 

I used the 305s on my Z51. I did go wider on the fronts as well. You will have no issues out of the 305s. Almost the same height as the OEM tire but a hair wider. Do look better from the back.

 

20 mm is .787 in. .4 inches on each side. How does the profile look? Maybe a bit more rim protection. Did someone mention they are 1.9 % taller? If so I believe thats not enough to throw off ABS etc. c but I would check that one. Can you post a picture?

 

They need to correct this at their cost if you did not authorize.

 

I have a call with the dealer this Monday since I was away all week.I'm sure he'll give me the option to keep the 305s or change to the 285s at his cost. Here are two pics of the 305s.

 

I think you will be fine with those tires.

Here are the things that should be considered when deciding to keep the tires:



Extra weight and larger diameter. The 305 tire is about 15% heavier which has slight effects on braking, accelerating, etc. The tire radius is about a quarter inch greater which if you travel on bumpy roads may be a little too much, there isn't a lot of clearance between the tire and inner wheel well liner. I know people will drop their cars more than that but heavy bumps even with the stock suspension height can cause interference issues. I know that I have issues with that diameter when running on the track and hitting the rumble strips at triple-digit speeds. If you are just driving on the street without hitting bumps at higher speeds you should be OK but there is less room for suspension travel.

The larger diameter may mean that TC will activate a little later than it would with the stock-size tires but TC, ABS and Stability Control should not be affected much by the larger diameter. The generally accepted rule is the systems can stand a 4% variation from stock size as long as the rear tire diameter stays the same or larger than the stock diameter. When the rear tire diameter becomes smaller than the front TC will activate even if you are coasting downhill in neutral.

Bill

 

I think I would go back to OEM if it was me.

 

That has been my thinking as well.
Unfortunately, at least in summer tire run flats the only option I see in OEM Z51 sized run flats is the Michelin Pilot super sports which were great in their day but now represent 14 year old technology.
Michelin decided not to support the C7 with its newer 4S tires in run flats. I'm hoping they will throw us a bone when, at some point soon I hope, they release their latest 5S summer tires in run flats and they can drop the old and far less relevant super sports
In the meantime, I'm on the newer AS4 run flats full time, but I want my UHP summer tire run flats.

 

^^^^^
The Super Sport runflats that came on our C7's were built to a special GM spec (tPc) that included some things we don't care about much- fuel economy was one of them.
If we buy PSS runflats now, they are built the spec that Michelin wants. That may be better or worse for most of us. I replaced the original PSS runflats with the current "generic" version last spring and am very happy with them.

 

The thing that stands out on that chart to me is how wide is the rear rim? OEM was good for 9.5-10.5", the 305 starts at 10.5".

 

Thank you to everyone for your comments and wisdom. Installer is going to replace the 305s with the original size 285s.

 

Probably the safest and best thing to do !!
Rich

 

So as spring approaches, I have renewed my search for Michelin Pilot Sport 4S runflats on stock Z51 wheels.
I am now finding the 245/35R19 front runflats available and virtually the same specs as the original equipment on my Z51 so no issues there.

However, the only MPS4S runflat option for the 285/30R20 rear stock size is 275/30R20.
Comparing specs stock 285/30 to 275/30 shows the following:

Measured rim size: stock 285/30 is optimized for 10" wheel while the 275/30 is optimized for 9.5 inches. However, the rim range on the 275/30 is listed as 9 - 10" so the tires should fit on my 10" wheels?
Section width: the same for both at 10.9"
Tread width: 10.4" vs 11.1" for the 275/30
Diameter: The same at 26.5"
Revs/mile: 770 vs 784 for 275/30

To summarize, the 275/30 4s is optimized for a 9.5" wheels but will fit on my 10" wheels. While the 275/30 has the same section width, the tread actually measures wider at 11.1" possibly a result of square rather than rounded shoulders; maybe similar contact patch? and the revs/mile are less than 2% different from stock 285/30.
Therefore, the smaller 275/30R20 should fit and perform very similar to the stock 285/30, without affecting the handling balance of the car. And they should play nice with traction control and Mag ride.

Seems logical to me. Am I missing something or can I expect these Pilot Sport 4S runflats to perform at least as well as my original equipment super sports on road trips, twisty mountain roads and the occasional HPDE? I may even consider taking a flyer on my first ever autocross as well.

What say you all?