Quote:
Originally Posted by BGTV8
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.
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.
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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?