Load rating 100 tires meet spec, why are 104 "required"?

[andycunn]

2022 California Route 1 AWD ER
Front GAWR: 2260 lbs
Rear GAWR: 3255 lbs

P225/60R18 standard load 100: 1764 lbs
Two tires per axle: 3528 lbs
Standard tires exceed the rating of the heaviest axle by nearly 300 lbs.

Can anybody point me to ford documentation clarifying their position on 104 load tires. Are they "recommended" or "required"? Does anybody have insight what specifically is wrong with standard tires that exceed the maximum gross axle weights? I've seen many enthusiastic claims - "standard tires can't handle the weight". But, they are rated to do exactly that.

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[AliRafiee]

How much weight can you put in the car? Especially the rear?

 

[andycunn]

How much weight can you put in the car? Especially the rear?

It's just a commuter and grocery getter. The maximum loaded weight, GVWR is 5760 lbs. The curb weight is reported to be 4727 lbs. So the max "payload" is just a bit over 1000 lbs.

Even with a 1000 lb load which I'll never get close to, even SL 100 tires still exceed the maximum rating of both axles. Per the GAWRs, the axles and car are past overloaded before standard SL tires are.

The point of my question is: Is there some benefit to XL tires other than weight carrying capacity? Is it a motor torque issue? Just "overengineering"? Something else?

 

[ChuckA]

It's just a commuter and grocery getter. The maximum loaded weight, GVWR is 5760 lbs. The curb weight is reported to be 4727 lbs. So the max "payload" is just a bit over 1000 lbs.

Even with a 1000 lb load which I'll never get close to, the tires still exceed the maximum rating of both axles. Per the GAWRs, the axles and car are past overloaded before even standard tires are.

The point of my question is: Is there some benefit to XL tires other than weight carrying capacity? Is it a motor torque issue? Just "overengineering"? Something else?

XL tires have reinforced sidewalls for safety and better stability in cornering. OEM tests various tires on their vehicles before recommending the XL. I’d take their word on it.

 

[Mach-Lee]

The load rating of the tires (times 2x) needs to exceed the highest GAWR. I'm not really sure what your question is, are you asking if you can use tires with a 100 load rating on the Mach-E? The answer is "in theory", but you would have to pump them up to 50 psi to carry GAWR. Which would be a bad idea, also considering they are not extra load rated either. The higher tire pressure would affect the traction and handling of the vehicle. So it's recommended that you do not do that and stick with tires that can handle the load at near factory psi.

Hopefully you know this, but the tires hold less than their weight rating at lower psi. So because the tire pressure spec is 39 psi and not 50 psi, one 104 rated tire can only hold about 1550 pounds at 39 psi. With a fully loaded rear axle, you'll be right at the limit of what the tires can carry.

You might be able to go down to a 103 rated tire safely (that's what I use), but I wouldn't go further than that. A lot of tire shops won't let you put on anything less than OEM spec.

 

[andycunn]

The load rating of the tires (times 2x) needs to exceed the highest GAWR. I'm not really sure what your question is, are you asking if you can use tires with a 100 load rating on the Mach-E? The answer is yes, but you would have to pump them up to 50 psi to carry GAWR. The higher tire pressure would affect the traction and handling of the vehicle. So it's recommended that you do not do that and stick with tires that can handle the load at near factory psi.

That's my point. Standard 100 tires do exceed the GAWR of the heaviest axle by a good margin. Yet Ford's door sticker calls for 104.

Maybe I have a bad table. According to:

https://static.squarespace.com/static/5395fbd3e4b003747ed3b60a/539a2d7ce4b0854264cd356d/539a2d82e4b0854264cd419b/1378330097907/Discount Tire inflation.pdf

SL 100 tires meet their maximum 1764 lb rating at 36 psi. Most tires allow a higher maximum of 50 psi but in SL anything over 36 psi deos not benefit the load capability. Less than 36psi does derate the load, as you point out, Lee.

104XLs do need more air to achieve the "extra load". At 36 psi they are derated to the same load as 100 SL tires. But at higher psi they capture a higher rating - 1881 lbs @39 psi in the table I found.

Hopefully you know this, but the tires hold less than their weight rating at lower psi. So because the tire pressure spec is 39 psi and not 50 psi, one 104 rated tire can only hold about 1550 pounds at 39 psi. With a fully loaded rear axle, you'll be right at the limit of what the tires can carry.

To my information I could run
100SL 39 psi - 1764 lbs load rating
or
104XL 39 psi - 1881 lbs load rating at that inflation

The 104XL has some more margin but both tires easily exceed half the GAWR at typical passenger car pressures. I'm just left confused as to the technical origin of the enthusaisim that XL tires are required.

 

[Mach-Lee]

That's my point. Standard 100 tires do exceed the GAWR of the heaviest axle by a good margin. Yet Ford's door sticker calls for 104.

Maybe I have a bad table. According to:

https://static.squarespace.com/static/5395fbd3e4b003747ed3b60a/539a2d7ce4b0854264cd356d/539a2d82e4b0854264cd419b/1378330097907/Discount Tire inflation.pdf

SL 100 tires meet their maximum 1764 lb rating at 36 psi. Most tires allow a higher maximum of 50 psi but in SL anything over 36 psi deos not benefit the load capability. Less than 36psi does derate the load, as you point out, Lee.

104XLs do need more air to achieve the "extra load". At 36 psi they are derated to the same load as 100 SL tires. But at higher psi they capture a higher rating - 1881 lbs @39 psi in the table I found.



To my information I could run
100SL 39 psi - 1764 lbs load rating
or
104XL 39 psi - 1881 lbs load rating at that inflation

The 104XL has some more margin but both tires easily exceed half the GAWR at typical passenger car pressures. I'm just left confused as to the technical origin of the enthusaisim that XL tires are required.

That table is 20 years old and I don't think it's right.

You base your inflation pressure on a) weight rating and b) max inflation pressure of the specific tire you're using. I looked, and most of the 100 standard load tires have a max inflation of either 44 psi or 51 psi. Load is proportional to pressure.

• If the 100SL tire has a 44 psi max inflation, you need to inflate them to 41 psi to handle rear GAWR.
• If the 100SL tire has a 51 psi max inflation, you need to inflate them to 47 psi to handle rear GAWR.

These pressures are very close to the maximum and don't give much room for error. Your figure of only 36 psi is incorrect and would lead to a dangerous overloading situation.

It is also a very bad idea to use a non-reinforced tire on an EV. Due to the weight, the flimsy sidewall will fold under hard cornering, and you'll have really bad handling and possibly a blowout. Insurance will eat you alive if you put standard tires on instead of XL reinforced and there was a serious accident as a result of poor handling.

 

[andycunn]

I think the origin of your inflation suggestion, Lee, is based on an assumption that a tire's rated load capacity is achieved only at the maximum sidewall pressure and the load capability declines roughly straight-line to zero at less than maximum pressure. That sounds sensible but I haven't found any standards body or manufacture published material to that scheme for passenger tires. I recall, perahps, that bias ply trailer tire inflations may closely fit a scheme like that.

Several manufacturer's websites publish identical information to my earlier table. They cite the "Tire and Rim Association" standards body, so these tables are unlikely to change with time. And there must be some invariant technical standard if a "load index" is to have any meaning at all, particularly when I need to buy tires for my 20 year old other car.

https://www.toyotires.com/media/pxcjubjs/application_of_load_inflation_tables_20200723.pdf

https://www.falkentire.com/load-inflation

Three of three sources I've found say 100SL carriers 1764 lbs, comfortably over the GAWR, at 36 psi, and can be inflated to 39 psi as that is under the maximum pressure but that extra 3 psi does not endow any additional load capacity.

The origin of this question is that I've noticed when shopping online for tires some websites list SL tires or better as compatible, most list only XL or better. Indeed I come to contradictory conclusions as well when going by GAWR versus the yellow sticker in the door.

 

[tuminatr]

Do Not install a 100 load rating tire on the MME. They do not support enough weight

 

[markboris]

^^^^ Being in the industry for many years, Brian knows tires. If he says you don't run a 100 load rating tire on the Mach-E, you don't.

When replacing tires on any vehicle, you don't look at the vehicles GAWR to determine what load rating your tire should be, you look at the load rating of the tire the manufacturer specifies for that vehicle. Driving hard into a turn or panic stopping will put much more pressure on a tire than just cruising down the road. XL tires have reinforced sidewalls, SL tires do not and every EV on the road comes with XL tires.

 

[ChuckA]

I was in administration at an independent tire dealer for 15 years. The rule-of-thumb is you can go up 1 or down 1 from the recommended speed/load rating for safety. Most dealers won’t deviate from this for liability insurance reasons.

 

[nicholsjon]

The largest load on tires is when you hit a pot hole or impact some sort of bump at speed. Those can be very brief loads, but can generate up to 4G loads into the tire (and suspension). Dynamic loading is what's important (as already mentioned; cornering, braking, etc) and impacts over road hazards. Tires need to be far stronger than just a static load such as GAWR.

 

[tuminatr]

I was in administration at an independent tire dealer for 15 years. The rule-of-thumb is you can go up 1 or down 1 from the recommended speed/load rating for safety. Most dealers won’t deviate from this for liability insurance reasons.

I would not go down in either. But I do agree dealers do this and yes there are some caveats. One of my former dealers was involved in a legal matter with a fatality. He had installed a lower speed rating on a car. OE was a V and he installed a T rated tire because it was cheaper. When he gave testimony he was asked if he understood the OE requirements. He said yes and he explained them to the customer and made him sign a waiver. The attorney asked who is the expert you or the customer? And followed up with why did you knowingly install a tire that didn't meet the specs. He lost the case.

There is no reason to take a risk as a consumer or business. If I was the shop owner I would refuse to install anything that doesn't meet OE Load requirements. Speed ratings I will make some exceptions for example my GTPE came with Y rated tires and it currently has V because those typically are considered performance speed ratings and the MME top speed is electronically limited.

 

[PalsyWalsy]

Despite what EVERYONE says here, I say go for it.....

 

[andycunn]

Do Not install a 100 load rating tire on the MME. They do not support enough weight

I agree that the 100 tire is rated less than 104. But both exceed GAWR so both appear sufficiently rated to support the weight. The 104 is more sufficient. I'm open to the idea that there is a reason for the 104 rating - but from published specs that reason isn't just "weight".

The largest load on tires is when you hit a pot hole or impact some sort of bump at speed. Those can be very brief loads, but can generate up to 4G loads into the tire (and suspension). Dynamic loading is what's important (as already mentioned; cornering, braking, etc) and impacts over road hazards. Tires need to be far stronger than just a static load such as GAWR.

At 4G the dynamic load on a fully laden car would briefly load the axle to 4 times the GAWR. If the tire must be speced for the dynamic load I would need tires rated 6510 lbs or more. Can I get dump truck tires in 225/60R18?

I believe DOT regulations for trucks and trailers are based on static weight and GAWR. Tucks and trailers hit potholes and bumps too. Perhaps Ford over-specs the tires on the assumption that a statistically normal Mach-E drivers will drive more exuberantly and hit potholes more exuberantly than a statistically normal truck driver. But this is just another speculative rationalization on my part.

Maybe it has something to do with regenerative braking? Regen brakes likely didn't exist when these tire standards and specs were made.

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