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Discussion Starter #1
Having ordered 18x7.5 45 offset wheels, I'm looking at tires for my '04 Wagon. Seem like the 'standard' options are 225/40/18 or 225/45/18. However, looking at _most_ tires available in those sizes the load rating isn't up to snuff with the 93 that is recommended by Mazda.

I really like the Kumho SPT that I have in 225/45/17 now - in 18's, they are:

92 XL for 225/40/18
91 XL for 225/45/18

This seems somewhat common for those sizes. Only a few are higher (Avon and Toyo come to mind).

What are those with wagons and 18's running for size and for load ratings?

For hatbacks and sedans, do Mazda also 'require' a 93 load rating?

What does XL really mean? eXtended Load, but does that just mean that an a non-XL tire's load rating is lower than the XL version?

I understand there are charts that show load rating for a given tire size and inflation pressure. I haven't been able to find any of those on-line. When I ordered my 17's from the Tire Rack they actually didn't place the order until they contacted me to say that I had to run at least 35 lbs. to match the factory 215/50/17 at 32 lbs.

From the Tire Rack, the load indices are:

93 - 1433 lbs
92 - 1389 lbs
91 - 1356 lbs

Am I right that those are at max pressure? Anyone (ToyoGuy?) have this info available?

Thanks much!

Kevin M.
 

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Kevin,

XL stands for Extra Load. The load index is given at max inflation pressure. The ones you have listed are correct, but for a standard load 93, then you would have 1433 at 36 psi, while the XL 93 would be 1433 at 42 psi.

Because the 6 was offered in other trim levels with tires bearing a 91 load index and because there aren't significant weight differences between the models, it is generally accepted that a 91 load index will work on any model of the 6.
 

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Discussion Starter #3
Thanks!

Looking at my owners manual, I get what you're saying - under the 'Standard Tire' spec it lists the 205/60/16 91H.

The manual also shows the GAWR (Gross Axle Weight Rating) as:

Sedan Front - 2359 lbs
Sedan Rear - 1958 lbs
Total (GVWR) = 4317lbs

Hatchback/Wagon Front - 2370 lbs
Hatchback/Wagon Rear - 2110 lbs
Total (GVWR) = 4480lbs

So, only 11 pounds different for the front, and 152 for the rear. Take the front of the wagon/2 gets you to 1185, still lower than the rating for a 91 at max pressure.

The question then is, what is the recommend pressure for an XL 225/40/18 or XL 225/45/18 to match the 32 for a 215/50/17? Can this be calculated, or is there a chart hiding somewhere?

Thanks again, toyoguy!

Kevin
 

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Discussion Starter #4
More searching gave even more interesting info - on the SEMA site:

http://www.sema.org/main/semaorghome.aspx?id=54434

There is a paragraph:

“On the aftermarket side, we’re recommending that you don’t change the overall diameter of the tire for Plus 1, Plus 2, Plus 3 and Plus 4 fitments because you’ll change the handling of the car,” said Knoper. “We recommend that you stay within 3% of the original-equipment overall diameter, and we also recommend that you stay within 3% of the original tire’s load index.”

Earl Knoper is apparently a consultant to Toyo (small world, eh toyoguy?)

So, that certainly matches what toyoguy said - I was getting frustrated that _all_ tire store and tire manufacturer sites I saw said 'thou shall not install a tire with a lower load rating than that on the placard'.
 

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Earl Knoper is apparently a consultant to Toyo (small world, eh toyoguy?)
[/b]
:laugh: Yeah, I know Earl! He was Senior VP of Marketing for Toyo for many years. He retired a couple of years ago and I think he now works for SCCA.

Check this sticky for recommended tire pressures:

http://forum.mazda6club.com/index.php?showtopic=33026
 

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Discussion Starter #6
:laugh: Yeah, I know Earl! He was Senior VP of Marketing for Toyo for many years. He retired a couple of years ago and I think he now works for SCCA.

Check this sticky for recommended tire pressures:

http://forum.mazda6club.com/index.php?showtopic=33026
[/b]

Thanks again - I'd forgotten that those numbers were there. I was hoping I'd find a chart so I could look at the number myself (there was a PDF file from Toyo in another country that showed part of such a chart.)

One last question - are those pressures universial across tire manufacturers? So, if I asked Kumho for a size vs. pressure load matrix would it be the same one you have?
 

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One last question - are those pressures universial across tire manufacturers? So, if I asked Kumho for a size vs. pressure load matrix would it be the same one you have?
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Yes. However, in many cases, the pressures given are for reinforced sizes. I may have to go back and specify different pressures for standard vs. reinforced, but that'll probably take a lot of time and research. The pressures I listed are on the high side for safety reasons.
 

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Discussion Starter #8
Thanks again!

Any other wagon/hatchback owners want to say what size and load rating they have on their 18" tires? BTW, are the factory 215/45/18's 93 load rated?
 
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