Mochilato

Just received my Signature Wheels SV104 in 20x9.5 et 55 and 21x12 et 45 and was recommended to go 265/30/20 and 325/25/21. The only tires that the rear has available are Pirelli PZERO and Michelin PS4S

I was calling around for tires and America Tire's said they can't sell me the tires because load of the car exceeds the limit of the Michelins. I saw that some in the forum do run 325s on 21x12s.

I just wanted to double confirm if its safe to ignore their concerns and find a shop that will install it regardless. Or do you guys recommend I go a different tire size? 2018 AMG GT C stock suspension.

TIA

AMG 17GT

https://www.tirerack.com/tires/tires...m=225YR1PS4SXL

XL stands for extra Load. Your GTG.

Stenzel-Germany

Hi,

I don't know the requirements / laws in the USA.

The european tire-load is coded as a number.
The speed index as a letter.

Your selected tire normally has the designation 102 (Y)

102 is the load index. This corresponds to:

850 kg / wheel
1700 kg / axle
3400 kg / vehicle.

Seems way too high. But the load specification is only valid up to 210 km/h.
If higher maximum speeds can be achieved, deductions must be made.

And you still have to consider the speed index.

Y means: max. 300km/h

(Y) means: also over 300km/h (but is not specified further)
Then always with higher tire pressure and reduced load capacity.

In German law, you can not fall below the load index and speed index entered in the approval.
Rear tire on a GTR has 106 (Y) from the factory.
102 (Y) would not be permitted (without a special test and with correction of the papers)


Best regards

Stenzel

AndrewFerg

I can't imagine anyone on this thread is going to recommend you ignore the load rating requirements. I would definitely not do that. It's so much more than the "load." In the case of a high performance car like this, it has to do with the structural integrity of the tire.

AMG 17GT

I remember putting on non-XL rated super sports back on an SL 10 years ago and it wiggled all over the place. After replacing with Xl’s, ride was great. The extra strength is in the side wall.

AndrewFerg

Exactly. Tires are spinning really fast at high speeds. Maybe 2,500 rpms? The stronger side walls help create the torsional rigidity as well as maintain the tires from flying apart. Just watch any drag race and the physics of what happens to those rear tires as they spin up. Now I've given four cents instead of two cents. Haha.

Skilly

325 25 21 is the only OEM diameter equivalent for a rear should you move to an aftermarket set up with a 21" rear. 106 vs 102 rating is about 220lbs per corner off spec, soo America's Tire and others of the like wont touch em. That said, there are literally 1000's of owners that have been doing this for all of their wheel set ups. So much so that there was a WW shortage for the better part of a year until just recently.

It's the same tire for the Huracan, R8, 911 etc. Any indie shop wouldn't think twice about setting you up.

AndrewFerg

Okay, I must be missing something in this conversation. I'm happy to be educated. Are you saying that the manufacturers of "Huracan, R8, 911" are selling cars with tires that are not rated for them? Not rated for high speed? That makes absolutely no sense to me.

G. P

Just to add a small detail that I think causes some "misalignment" here.
Mochilato has a 2018 GTC (not GTR). The standard rear tires on GTC are 305/20/30 (103Y), so the difference between the 102Y and 103Y is only 55lb per wheel.

Skilly

You are.

What I am saying is that EVERY aftermarket wheel manufacturer you can think of sells a 20/21 set up for all of those vehicles - HRE, AL13, AG, etc...Butler Tire, Wheelsboutique (a sponsor on this forum) etc install this set up, and with their volume its likely almost a daily thing.

Also 102, 103 and 106 tires are speed rated the same (rating Y); it's the overall weight capability that is a different.

Skilly

Just for fun, I just did the rough math...a full rotation on the OD of these tires has a travel distance of around 7'4"...so this is a little over 205 mph.

dlefty

OP-don’t worry or overthink it, Michelin’s 325/25-21 will be more than fine, it’s the tire that has been run by a majority of GT platform cars that go to a 21” rear aftermarket wheel for many years now.

Mochilato

Thank you all for the discussion. I won’t be tracking the car anytime soon, if that’s help in any way.

I was more in the position, am I screwed for not worrying about load when upgrading my wheels. However like some have stated, many GT vehicles have this setup. So that’s some peace of mind.

The rears are usually hard to source due to the above, but surprisingly enough, it was the front that were harder for me . Wheels are going on this week!

G. P

The discussion is focused on the rear tire, but I see the issue with the front tires. Here is the rational:

One thing a manufacturer is considering about Max Load rating when deciding on the OE tire is the load margin needed with driving dynamics. So here is a simplified example using the AMG GTC.

A 2022 AMG GTC with full tank of gas and about 200 Lb driver with some light carry-on items has a total weight of about 4060 Lb:

1860 Lb on the front axle and 2200 Lb on the rear.

The OE Tires are F: 265/35-19 (98Y) 1653 Lb max load and R: 305/30-20 (103Y) 1929 Lb max load.

In a stationary state or constant cruising speed, each of the front wheels carries about 930 Lb and each of the rear wheels about 1100 Lb. This is well within the safety margin of a (103Y) tire.

However, when breaking hard in a straight line, the weight transfer from the rear to the front can be anywhere from 40 to 60%. In a 50% weight transfer example, 1100 Lb of the rear’s 2200 Lb weight will be transferred to the front, adding 550 Lb to each front tire load. This is within the safe margin of a (98Y) tile, but not a (93Y) tire suggested by the OP. PS4S 265/30-20 is (93Y) with max load of 1477 Lb which is below the safety margin.

This situation will get worst if you plan to track the car.
For example, in a tail-break situation coming at high-speed from a straight line into a curve, you may see a 40% rear to front weight transfer for breaking hard together with about 30% for steering into the curve for inside corner to outside corner weight transfer. Using the above car weight there will be additional 860 Lb added to the inside front tire load which already carries 930 Lb, to a total of 1790 Lb, much above a (93Y) rating and even above (98Y) OEM. Therefore its better to use a CUP2’s tires on the track. A 275/35-19 (100Y), which fit the OE GTC front rim, has a max load of 1764 Lb, close enough to the virtual load created on that front tire.

My 2 cents 😊

P.S. I just want to indicate that there are many variables that will affect the weight transfer calculations, such as center of gravity, chassis stiffness, shocks, coils and other chassis setup. Tire temp and pressures to name a few. but the above should give you some idea of what need to be looked into.