Housey

Hello,

I posted this in the GT forum, but have not managed to get any responses so thought I might try the forum for tyres (mods please remove the other thread if double posting is frowned upon).

I have a 2018 AMG GTR running the factory Cup 2 tyres which I have covered around 3500 miles on. The challenge, which most will be aware of in the UK, is wet weather performance so I am looking at options to replace them for daily use, keeping the Cup 2's for track use only. I have seen a few threads but nothing current and frankly concise, so thought I would start another. My car runs the lightweight wheels as you can see here.


Current tyres are UK spec and standard fit Michelin Cup 2 in following spec.
Front: 275/35/ZR19 100Y
Rear: 325/30/ZR20 106Y

PS4S is the obvious choice as they are a superb tyre I have run on other cars, but of course not available in the 325 size. So here are my questions...

1) Can I move to 335/30/ZR20 101Y (Note both width AND load rating)
2) Can I move to 345/30/ZR20 106Y (To get the load rating)
3) Should I up the front tyres by 10mm or 20mm to match if I do
4) Has anyone done this on a GTR and have you had any issues with sensors

Additionally, Mercedes also has the 265/35/R19 98 front 295/30 R20 101 rear option on the filler cap and manual and wondered if anyone has downsized to these on the above wheels and would welcome any views if you have.

Thanks in advance for help and hope this helps a few with a similar dilemma.

Acta_Non_Verba

Allow me to offer a little info -
Whenever I venture outside the boundaries of factory specified tire sizes on modern cars with ABS and skid controls, I like to compare the rolling circumferences as a matter of how the differences impact axle speeds. It is differences in axle speeds that systems such as ABS and ESP use as one of their controlling parameters. What I don't know with this car (or others for that matter) is where the factory threshold is, so using the comparison has its gray areas. Factories don't typically publish such #s, but at some point if I am seriously different in axle speeds, either accelerating or braking, these systems might interpret the differences in speeds as a driver who has lost control. I just want to make sure I am somewhat close to factory deltas; otherwise, if it is seriously different (5% over starts to become a question mark for me) I wouldn't consider those sizes.

To determine deltas I use a tire size calculator like this one:
Tire Size Calculator

A couple of screen captures - the factory sized GTR tires show a 4.1% difference between front and rear axles:


If I use the larger of the two rear options mentioned then I get this difference, a much larger 5.6% delta.



Again, whether the 5.6% difference makes any negative impact at all, I wouldn't know. Maybe OK, maybe not.

On at least one other car with which I had experience, a Bentley Continental GT that specified a "square" 19" wheel size at all four corners, the aftermarket offered staggered wheels, some sizes of which would trigger the ABS on moderate stops (not aggressive). So, unfortunately, it leaves you in "experimental mode" if you want to try these sizes.

One other comment, on your last line about the smaller sizes - 265/35/19, and 295/30/20 - these are the factory specced tires for the AMG GT. The GTR is a different wheel base, different wheels, etc. I don't think those tire sizes were meant as an option for the GTR.

Hope any of this helps.