Need some advice on tires

tibor370

Hi guys,

I'm going from 18" wheels to 19" wheels and I need some advice.

My car (used) came with 245/45 18" in the rear and 225/45 18" in the front. This works out to a 678mm diameter in the rear and 660mm diameter in the front, so a 2.7% difference in diameter.

From factory, the car had 245/45 18" in the rear and 225/50 18" in the front, which works out to a diameter of 678mm and 683mm respectively, so from factory the difference in diameter was just 0.7%.

And the other difference is that from factory, the diameter of the rear tires was smaller compared to front tires, whereas on my car it's the other way around, with the rear tires having an 18mm larger diameter than the front tires.

I'm really struggling with rear-end traction under heavy acceleration which is obviously going to be due to the fact that the rear tires are quite narrow at 245, but could the front/rear diameter difference be part of the issue as well?

The reason why I'm asking is because I bought the original 19" rays rims and they came with Michelin PS4 S tires on them - 255/35 front and 285/35 rear. The rear tires need to be replaced but the front tires still have some life left in them.

I want to replace the rear tires with 275/35 Michelin PS4 S and eventually the front tires with 245/40, so the factory 19" set-up. I want to go with stock tire sizes due to bureaucratic/regulatory reasons (I'm based in the EU).

But I'm wondering whether I could keep the 255/35 in the front for a little while and combine them with 275/35 in the rear?

The problem is that with this set-up, the difference in diameter would be 2.4% and like now on my 18s", the rear tire diameter would be larger than the diameter of the front tires. Whereas the factory 245/40 front + 275/35 rear 19" set-up has a diameter difference of just 0.3% with the rear tires having a smaller diameter than the front.

Am I correct in thinking that having larger diameter tires in the rear vs. the front is not ideal and means that the weight of the car is shifted more to the front, which could result in less rear-end traction under acceleration?

Should I just replace all 4 tires and go with 245/40 front + 275/35 rear and avoid running 255/35 in the front together with 275/35 in the rear due to the diameter difference? Or does it not make that much difference?

jchammond

255/35/19’s up front 26.0”
295/30/19 rear 26.1”
The 295’s are nearly 2” wider than the 255’s but will fit Sport wheels, or 10” minimum

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tibor370

Thanks for the reply, but like I mentioned, the plan is to go with the stock 19" set-up eventually (275/35 rear and 245/40 front). I live in a country where cars need to pass an inspection every 2nd year and one of the things that are checked is the tire size, which needs to match the manufacturer's specifications. Getting non-standard tire size approved is a nightmare.

I just need to know whether it would be ok to go with 275/35 rear and 255/35 front for a little while before I replace the front tires with 245/40.

jchammond

My guess is that you would get the Christmas Tree effect, all kinds of lights popping on.
I’ve always heard stay within 3%, but best to stay within 2.
Depends on brand of tires; how much thread is left on the front, are the 275’s gonna be 26.6” or (more/less).
It’ll be close….. but I wouldn’t exceed 1/2” difference between the 2.
Sport sizes still have negative rake like the base…only Nismo sizes have slightly taller rear tires from the factory

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tibor370

Thanks for the reply. Like I mentioned I'm currently on 18s with 225/45 in the front and 245/45 in the rear, which is a 2.7% diameter difference and I'm not having any issues with the electronics.

So 255/35 front and 275/35 rear would actually be slightly better than my current set-up with "only" a 2.4% difference in diameter.

But I think I'll just replace all 4 tires right away and go with 245/40 and 275/35 to avoid any issues.

So can it be actually advantageous to have larger diameter tires in the rear compared to the front? Or does that lead to handling/traction problems?

I know that some supercars (like the Aventador) have significantly larger diameter tires in the rear vs. the front.

Why is it such a problem on our cars then? Is it just the way the electronics have been set up and there is nothing inherently wrong with running larger diameter tires in the rear?

cv129

I had 265/35/19 fronts, 285/35/19 rear for a few years, my car was ok with that, didn’t give me Xmas tree lights.

I currently have 275/35 and 285/35

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tibor370

So I just found out that Michelin Pilot Sport 4 S are only available in 245/40 19" size as run-flats. WTF Michelin?

I am currently deciding between going with Michelin Pilot Sport 4 (I currently have them on my 18" wheels and have no complaints) or I'm tempted to try the new Bridgestone Potenza Sport, it's getting really good reviews and seems to be on par with Michelin Pilot Sport 4s, even beating the PS4S by a small margin in some areas. My only concern with the Potenzas is the wear, my Michelin PS4 have been excellent in this regard.

Any other suggestions?