Replacement Tires and Load Ratings

radensb

It's that time. Michelin Pilot Super Sport seems to be well received, so Im going with them in the stock size.

I am a bit confused about the Load Range parameter on tires. America's Tire shows that an OEM matched set for the Z is XL (Extra Load) in the front and SL (Standard Load) in the back. The others show SL in the front and XL in the back, so i dont know if that is a mistake or not. If anything, I would think that you want XL in the back and SL in the front. I consistently read that XL are for higher performance, but the SL ones are more expensive (~$30/tire). Seems counter intuitive. I dont really mind the feel of the XL's that I have been using, but I wanted to get some feedback regarding what I should get, and the pros and cons for our car and see what load ratings others are using.

On the site, if I select the front and rears separate, I can select SL or XL for the rears. Should I go with America's Tire recommendation, or get all XL's?

I do not track. I do not do burnouts. I may hit the backroads every once and a while to play, but nothing too punishing or for very long. Still, I would like to make sure I really enjoy the driving experience, and paying a bit more for a tire that can provide that is worth it to me.

Thanks!

barncobob

call Tirerack, they have reps to help

BGTV8

Do not worry about the XL rating ......... what you need to work off is the load index which is the 2 or 3 digit code followed by an alpha after the size.

The Z34 requires 95W front and 96W rear "load rating".

My 2009 with 18" wheels has 225/50R18 95W and 245/45R18 96W as standard.

An "XL" tyre simply says that the load is influenced by tyre pressure.

For an explanation of "load index" - see here:
Tyre Markings - Bridgestone Australia

The "XL" designation is explained here:
Web theft yields the following...

Extra Load tyres are designed to be used at higher pressures than normal tyres. They have the same maximum load capacity as a normal tyre with the same Load Index but achieve this at a higher pressure. Normal tyres are inflated to pressures between 2 bar and 2.5 bar. They reach the maximum load capacity as specified by the Load Index when inflated to 2.5 bar (35psi) Eg for a Load Index of 94 this is 670kg. Extra Load tyres can be inflated to pressures up to 2.9 bar (41psi) and actually have a lower load capacity at the same pressure as a normal tyre with the same Load Index. To get the 670kg load capacity for a Load Index of 94, an Extra Load tyre must be inflated to 2.9 bar. At 2.5bar it will only have a load capacity of 595kg

So, and XL tyre is actually less capable that a non-XL tyre as they achieve compliance with their actual load index at maximum permitted pressure.

radensb

I believe I read something similar to this as well, but then I also read that the load index and load range are unrelated. Load index (100Y for example) is a measurement of loading capability at pressure and load range (SL/non-XL, XL) is a description of construction and sidewall reinforcement. I found this article for the GT-R, that recommends XL tires, mainly due to the fact that the reinforced sidewalls will cause less sidewall deflection when perpendicular forces are applied (ie: making a turn). It suggests that all sports cars would benefit from them.

I have also read a few posts from people who get new tires and hate them because they feel they make the car more unstable while others love them. I wonder if people in these cases had XL and inadvertently switched to an SL (or non-XL) without knowing and experienced greater sidewall deflections and blamed the tire?

As I understand it, as long as the load index meets or exceeds the OEM and the load range meets or exceeds the OEM (perhaps with added tire pressure needed in the XL case), the tire should be compliant with the vehicle requirements, correct?

I know that the Michelin PSS 245/40R19 size only comes with an XL rating, so I have no choice for the front tires. However, the rear size (275/35R19) is available in both SL and XL. So I feel like I should keep them the same and enjoy the added stability.

I am a bit surprised there isn't more discussion on this topic. Many, many people (including on this forum) have gone the PSS route. I wonder what they chose and how its working out for them? I currently have XL in front and SL in the rear and always felt the back end could be tighter. Maybe that's why?

radensb

Well, unfortunately this thread didnt get more "traction" (...sorry), but I did learn some stuff and figured I'd share in case others had questions or were curious.

I ended up getting the Michelin Pilot Super Sport. I got XL's all around. All I can say is WOW. What a difference from the worn out, NANKANG tires that were on the car when I got it. The PSS looks great and handling is night and day better. And they are QUIET! Huge improvement. I can actually use bluetooth in my car now. They are a bit loose on traction, but that is expected with new tires until they are broken in. I expect that to get much better.

With regards to the XL vs SL matter for our cars, the Z34 does not need the XL rating. Factory tires are SL rated, however, MFG's do not make SL in all sizes. This is what lead to my initial confusion. For Michelin PSS, the stock front tire size only comes in XL, while the rear size come in both. That's why the auto-matching on sites like Tire Rack and America's Tire selected XL's for fronts and SL for the rears. When I checked other tires and brands, I saw SL/SL, SL/XL, XL/XL combinations, so it was clear that any combination was acceptable. However, the recommendations I got were to keep the Load Ranges consistent between the front and backs. While mixing them are OK and still within OEM spec, it will change the handling and feel of the car as the fronts will be more ringed than the rears. This may not have been a huge deal, or even noticeable, but I didnt want to take a chance and Im glad I didn't. Going from XL/SL combo to XL/XL is a definite improvement to me. Sure, its also a way better tire in general, but the back doesn't sway as much. The XL's have a higher load index at a higher pressure than the SL's (35PSI vs 41PSI), but even at 35psi, you are still within OEM load specs of 94 and 96 respectively as the load capacity to inflation pressure is linear.

Stock Tires (from door jam):
Fronts: 94W (Rated at 1477lbs @ 35psi)
Rears: 96W (Rated at 1565lbs @ 35psi)

MPSS:
Fronts: 98Y (Rated at 1653lbs @ 41psi)
Rears: 100Y (Rated at 1764lbs @ 41psi)

XL's have a higher load capacity at a higher pressure of 41psi, but have the same load capacity as an SL tire at 35psi. (source)

In general, what the XL's provide is a more robust, more ridged tire with reinforced sidewalls that improve cornering performance by reducing tire deflection and are more resilient to road hazards. They are also a bit heaver, which is expected due to the added materials to make them more robust and handle the extra pressue. Also, in the case for the MPSS 275/35R19's, they were about $35 cheaper. Oddly, the above source claims that XL's actually hurt performance as they are thicker and retain heat. However, not by any noticeable difference. Its also the only source that I have read that claims that. Everything else I have read states that performance is better, especially for sports cars, and from what I have experienced so far, they are correct. Perhaps this varies from vehicle type?