load capacity with 35 vs 44 psi tires

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390
Location
Greenville SC
My car came with 205/70-15 tires (97T). The sidewall says 1609 [email protected] psi. The car sticker says inflate to 35 psi. The replacement tires are 205/70-15 (also 97T). The sidewall says 1609 [email protected] psi. To get the same load capacity, am I supposed to inflate the new tires to the higher pressure? Is there a source (even if for one manufacturer and one tire style) that relates load capacity to inflation pressure?
 
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3,939
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Somewhere in the US
First, If you look carefully, I think your new tires will say 1609 # max AND 44 psi max (Not "at".) There are several tire standardization bodies throughout the world. In the US it's the Tire and Rim Association (TRA). In Europe, it's the European Tire and Rim Technical Organization (ETRTO). These folks set the load vs inflation pressure curves. The load curves are size specific - that is for a given size there is a curve and it is unique to that size. However, there has been some standardization (that's what they are supposed to do) where tires with similar load capacities now have the same load capacity. TRA and ETRTO sizing standards are slightly different (aside from the metric vs english dimensions). TRA puts a letter in front of their sizes where ETRTO does not. Plus (due to the metric thing) TRA P metric tires have a maximum pressure of 35 psi for standard load and ETRTO has a maximum of 2.5 bar (36 psi) for standard load passenger car tires. HOWEVER (and this is where your question gets answered), there are ceratin circumstances where using more inflation pressure is called for - high speed operation for one. ETRTO called for optional maximum pressures of 3.0 or 3.5 bar (44 spi and 51 psi). As first steps in having a world wide tire standard, TRA has adopted these optional maximum pressures. (Technically, this is exactly right - more inflation pressure for high speed operation is the same regardless of where you are in the world.) HOWEVER, the load curve is unchanged. It ends at 35 psi (or 36 psi) if you have a standard load tire. Hope this helps.
 

George Bynum

Thread starter
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390
Location
Greenville SC
OK, I'm 99% sure I follow you. So at 60MPH, a 35 and 44 rated tire would have approximately the same load rating at 26 psig (or at 35 psig). The 44 rated tire would be better suited at 100 mph because there is "room" to add some pressure. I assume that the organizations you mention don't make their publications available on the web, right? They have to earn a living too. (Yes, I do know that "Google is your friend", but am answering before looking) Thanks!
 
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688
Location
Fort Smith, AR
CapriRacer is right. Here is what Yokahoma has to say about the subject: "The load-carrying capacity of P-metric tires is rated either Standard Load or Extra Load. Standard Load tires are limited by the load that can be carried with a maximum inflation pressure of 35 psi. A Standard Load tire may be branded with a maximum inflation of 44 psi, indicating the tire's ability to meet special performance requirements. It does NOT increase the tire's load capacity. The maximum pressure for a tire is always indicated on the sidewall. This maximum pressure should NOT be confused with the vehicle manufacturer's recommended tire pressure. The vehicle manufacturer information is listed on the Tire Information Placard located on the door post or glove box lip, and is also contained in the Owners Manual. ALWAYS follow the vehicle manufacturer's information." I think the more important consideration is that the 205/70R15 tires are the correct size specified by your car manufacturer. If so, your tire's load capacity should be safe if you maintain at least 35psi. Increasing the psi to 44 on a SL tire probably gains no load capacity but may adversely affect handling by creating a "balloon" feel when driving from over inflated tires...which may be 1 reason why the manufacturer wants you to stay at 35 psi. I'm no expert, but I think you actually do lower the load capacity by lowering the air pressure to 26 psi as you say, even though increasing it above 35 psi does not increase the load capacity.
 
Messages
3,939
Location
Somewhere in the US
George Bynum said: "The 44 rated tire would be better suited at 100 mph because there is "room" to add some pressure." Actually, no, the pressure says nothing about speed suitability - speed rating does! It's just that some tire manufacturers have different interpretations of what the law says relative to what is supposed to be printed on the sidewall. "I assume that the organizations you mention don't make their publications available on the web, right? They have to earn a living too." Exactly. These organizations are non-profit, but they still have to have an income for the expenses involved in organizing and publishing this info. The books are fairly expensive (about $50 each) and unless you are a tire manufacturer it makes little sense to have one. Roger said: "I'm no expert, but I think you actually do lower the load capacity by lowering the air pressure to 26 psi as you say, even though increasing it above 35 psi does not increase the load capacity." Expert or not, you're exactly right - lowering the inflation pressure lowers the load capacity of a tire. The reason for the increased inflation pressure above 35 psi not resulting in increased load capacity is that this increase in pressure is supposed to be for reduced high speed heat generation. Hope this helps.
 
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