"XL" rating for car tires 1

[TheBlacksmith]

I have a Focus ST with 235/40-18XL tires. It is ungoverned and drag limited to about 150 mph, not that I intend to drive that fast. I can find plenty of talks about not using a lower SPEED rated tire, even if you don't drive that fast because of adverse affects on handling in how the tire reacts with the suspension. Is the XL rating, affecting sidewall strength/stiffness the same? The XL rating severely limits my replacement tire choices. I always thought XL was more of a light truck P-rated or LT tire concern. While I may never approach 150, many of you may know I love carving up the twisties.

 

[CapriRacer]

Mr. Smith,

There is an "engineering" answer to your question - and then there is the every day good advice type of answer. I'm going to give you the latter, but if you want to talk in depth about the relationship between speed ratings and load ratings, just ask.

For practical purposes, the Extra Load (XL) rating is about load carrying capacity. You will find it very difficult to get anyone to mount Standard Load (SL) tires on a vehicle that calls for XL. Not following the vehicle manufacturer's recommendation opens a whole lot of legal liability that most tire shops do not want to get into.

Many cars use XL rated tires - and I've always interpreted that as an effort to avoid having to redesign the fender to accommodate a larger tire.

Passenger car tires are done a bit differently than truck tires. In truck tires they have "Load Ranges" - and they use letters - with letters further into the alphabet indicating higher load carrying capacity. In passenger car tires, they use just SL and XL.

Handling? Even though handling is commonly talked about as the "reason" why you should not use a lower speed rating, it is a bit of misdirection. As a general rule, higher speed rated tires handle better, but that is not always the case. You have to understand that there are legal liability situations that preclude doing things that otherwise would be OK. Like using a 150 mph speed rated tire on a vehicle where the speed limit is 80 mph. Although this sounds quite rational, there are plenty of sharp lawyers out there that can take advantage of such a slip in judgement on the part of a tire shop.

Practical advice? I think you will find any tire shop worth working with will insist on using XL tires. I also think that those shops will also insistent on using at least the speed rating on the vehicle tire placard.

I hope that helps.

 

[TheBlacksmith]

Thank you, I suspected that was the case. I am familiar with truck tire load ranges. However my F150 pickup comes standard with P235/70-16XL; it makes sense to use an XL on a pickup I just hadn't seen it before on a car. My older Focus SES was fitted with high speed 205/50-16SL tires and I found a replacement all season tire that served me well until I bought the new car. That particular tire is available in the 235/40-18 size and was one reason I bought the particular car I did, thinking I could use that tire again as a replacement.

 

[CapriRacer]

It's obvious from both Mr. Smith's and Mr. Peterson's response, I need to explain a bit further.

First, the load tables for SL and XL are identical - EXCEPT the XL can take higher inflation pressures and that results in higher load carrying capacity.

Second, a tire on a pickup can be an XL, but if a larger SL tire was desired, it could be used if it had the same or more load carrying capacity. It used to be fairly common for 1/2 ton pickups to come with XL tires, but the trend is towards larger, but SL tires.

In theory, an LT like tire with Load Ranges COULD be used for a passenger car. The net effect would be a smaller tire (in physical dimensions), which would be easier to package, although the spring stiffness of the tire would cause some headaches in the ride department. Side note: The idea of using a smaller but more highly inflated passenger car tire which could get improved rolling resistance is out there. I believe Bridgestone has a concept that they have offered to some of the OEM's.

And lastly to Mr. Smith: I want you to look very, very carefully at the vehicle tire placard, which should be on the driver's door frame. EXACTLY what does it says there? Be sure to include the Load index and speed rating. If it says 95 for the Load Index, then that is an XL, but if it says 91, it's an SL.

Looking at the other Foci (or is that Focuses?), they require a Load Index in the vicinity of 88, and a 235/40R18XL has a Load Index of 95, but a 235/40R18SL has a Load Index of 91. So I wonder if Ford merely supplied an XL, but specifies a SL.

On the other hand, I went to Tire Rack and out of 72 possible tires in that size (V speed rating and higher), 44 were XL. So I'm not seeing that as a huge deal.

 

[TheBlacksmith]

CapriRacer - Yes it is a 95. When the time comes I'll just pick one of the 44 brands and probably be very happy. Obviously my previous favorite high performance all season tire is not in play. Thank you all for the discussion.

 

[NormPeterson]

Star for that, CapriRacer, and I've redflagged my previous post. The Toyo .pdf I'd found had listed SL and "Reinforced" tables, and apparently ETRTO "Reinforced" is not the same as XL. I did find another, more difficult to use online .pdf that may let me fill in some or most of the XL table in a format easier to use.


Norm

 

[CapriRacer]

Norm,

You have to be careful about this area - and I'll point out why.

ETRTO handles displaying the load vs inflation pressure differently than TRA does. ETRTO does it by Load Index. So they have a table for SL tires and they have another table for "Reinforced" tires (the equivalent to XL). The problem is that with the TRA chart it is obvious, and with the ETRTO charts, it is obscured.

But if you look very very carefully, the chart on page 5 says exactly what I told you.

And in fact "Reinforced" and "Extra Load" have identical meanings.

Now you may ask, why don't the tire manufacturers (through the tire standards organizations) eliminate the confusion and the differences. - to which I say:

First, the charts were created a long time ago when the world was much larger than it is today and there was little overlap between regions. And once it is published, you can't take it back.

Second, they are indeed working on consolidating the load tables. But the existing tire sizes can't be changed, so there is a built in problem. I'm sure the next time there is a change in the way tires are sized, there will be a single global standard.

 

[NormPeterson]

Thanks, Capri.

I'd spent quite a bit of time with that document and the tables that it contains, and having so many different load rating tables is confusing at best. At least this wasn't getting into P-metric tables, which from what I've seen (and try to avoid) are a real hodge-podge.

It appears that any given load index line for SL correlates to the XL for four load index numbers higher, in which case I think I'm seeing it straight. Relating two different LI's to different tires of the same size is probably where I'm having the greatest difficulty.

Thanks for clearing up "XL" vs "Reinforced" - I assume that this means I can treat the terms interchangeably as long as I stick with ETRTO sizes rather than P-metrics. I'd thought that was the case but had been getting second thoughts, especially after seeing a paper by John W Daws that gets into stiffness and deflection under load and raises a few questions I'm not sure I can yet answer even to myself.

FWIW, the whole "different size replacement tires" is an interesting conversion that I'd thrown at Excel specifically for situations like Blacksmith's, which I see from time to time on enthusiast message boards.


Norm

 

[TheBlacksmith]

I'm glad I was able to provoke a bit of a professional debate/exchange in addition to getting my question answered. I sort of knew the answer and was disappointed that the tire brand I had excellent results with in the past didn't make the 235/40-18 in an XL. At the risk of starting another debate, I do like the new premium rating of AA traction, so the TireRack selection is really about 6. I'll just give my money to Charles or Bibendum and I'm sure be very happy.