Load Index

Discussion in 'SRT Hellcat Wheels and Tires' started by Tire God, Apr 8, 2016.

  1. Tire God

    Tire God Hellcat Member

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    If the door placard says 32psi. And the OE tire is a 275/40-20 106XL, then you calculate the weight capacity with the load inflation tables to see what the lowest one you can use is. A 106XL at 32 psi carries 1,680 lbs. The pressure ceiling for weight capacity of an extra load tire is 42psi and for a standard load it is 36psi. But the 99 is what determines the weight, the construction type just changes at what pressures the tires have to be to carry the same weight. So you can go all the way down to a 99XL or 99SL. The 99 carries 1,709lbs at the pressure ceiling. For a 99XL you would have to run the tires at 42 psi and for a 99SL you would have to run 36psi. But remember the TPMS is programmed to turn on after a 20% drop from placard pressure. So if it's 32, it probably won't come on until 26 psi. But if you need to run 42psi for a 99XL that is dangerous because it's still programed for the 32 psi.
     
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  2. BULL

    BULL ________173.7mph_______ ___2016 Colorado Mile___ Gold Supporting Member

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    What is the context in which you've provided this statement?
     
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  3. Tire God

    Tire God Hellcat Member

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    I have seen multiple conflicting posts and confused people, like saying you can't run a load index lower than 106XL. Just trying to explain what it is and how it works. Hopefully it's helpful.
     
  4. BULL

    BULL ________173.7mph_______ ___2016 Colorado Mile___ Gold Supporting Member

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    There's lots of potential for usefulness and it's appreciated.

    Can you explain the calculations and provide some source material? I'd hate for someone to use the information in an incorrect fashion and "hurt" themselves or others...


    ; )
     
  5. Tire God

    Tire God Hellcat Member

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    Toyo has it on their website.
    Load & Inflation Tables | Toyo Tires

    Just remember that the P-metric and Euro metric sizing is actually the same. All you do is add 36 psi instead of 35 psi for the pressure ceiling for standard load and 42 instead of 41psi for extra load, and its actually the same thing. A P275/40-20 105XL will show as a pressure ceiling of 35psi due to the strange way Americans adopted the tires. But it's the same as a 275/40-20 106XL when you add 1 more psi to make it 36psi. This is the confusing part most people don't understand. P-metric is American while euro metric (or nothing in front of the size) is from Europe and was adopted as well as the speed ratings and such. The European people have always been ahead of us lol. And the door placard is what the tires being installed has to meet or exceed for load capacity. Another note, the max pressure you read on the sidewall actually has no impact on load capacity. Some vehicles will have a placard that says 42psi front and 48 rear for example. They don't carry any more weight after 42, the rest of the pressure is so it rides and handles like they want.
     
  6. BD2DBONE

    BD2DBONE Silver Member

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    Tire God - for the ADD folks, what tire pressure should we maintain on the stock tires. Have summer if there's a difference. Also, would traction improve if I lower pressure and what's acceptable without creating risk? You seem to be living up to your name. Thank you.
     
  7. Tire God

    Tire God Hellcat Member

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    Knowing the answer to that is like knowing the answer to the meaning of life. All I can recommend for the stock tires is what's on the door sticker. The way to figure it out is not easy. You would have to constantly monitor the ambient temp outside, the street or track temp, and run the tires hard and stop every so often and measure the tires temp with an pyrometer to see at what ambient temp, street temp and what air pressures get the tires to get the most evenly heated across the whole tread surface without over heating the tires. That is the formula that professional racers use like in Nascar and Formula One. There is no simple answer. Also to figure out what is the lowest safe air pressure, there is a sticker not in the door jam but on the door itself, should be labled F.M.V.S.S. It will list the GAWR (gross axel weight rating) and the GVWR (gross vehicle weight rating) and there would be a very long series of calculations regarding thoes numbers and the curb weight with only you in it, and the reserve load, which is a safety cushion that was implemented by the DOT. The short easy answer is, if you want a tire with more adhesion, buy one. Thoes PZERO ultra high performance summer tires are great. But if you want something to really stick to the ground, get a michelin sport cup, or if that isn't sticky enough, nitto NT01, or if that isn't good enough a Hoosier R7. But realize as the more traction you have with less tread, there will be no wet traction, especially with the Hoosier, and you will be lucky to get 5k miles on a set. So it's all about what's most important. No one tire does it all (like my other post), you just have to decide what is most important, traction? 3 season ability? Better longevity? Ect. Or just have 2 or 3 sets of tires on rims and put them on accordingly, if you can afford it haha. I know I couldn't. And I hope I'm living up to my name. I just wish there was one forum where I could put all the info for everyone to see. I can't make accounts on 30 different ones. I don't have the time. So please share all of this knowledge that I have brought to the Hellcat forum.
     
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  8. GBTXHellcat

    GBTXHellcat Hellcat Member

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    So if I want to Run a Pirelli 104y in 305/35 zr20 on my 10.5 inch Triumph wheels that are being made now I can as long as I run it slightly over pressure ?
     
  9. Tire God

    Tire God Hellcat Member

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    Correct. A 106XL at 32 psi carries 1,675lbs. A 104XL carries 1,587lbs at 32 psi, but at 35psi it carries 1,709lbs. And a 104SL carries 1,731lbs at 30psi. Click on the link and click the load inflation tables and scroll to the bottom. You can print it if that helps. It shows every load index and what it carries per pound of pressure. It's split between standard load (SL) and extra load (XL). Easy to read too.
     
  10. Tire God

    Tire God Hellcat Member

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    Whatever you put on, it just has to meet or exceed the vehicle placard. 275/40-20 106XL at 32psi=1,675lbs. As long as the load index can meet or exceed the weight capacity specified by the door placard(1,675lbs), its safe.
     
    Last edited: Apr 16, 2016
  11. Sammy123

    Sammy123 Active Hellcat Member

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    The Nitto NT555R drag radials are rated at 102V in stock tire form (275/40/20). What would the psi have to be? Can they be kept safely still at 32psi?
     
  12. Tire God 2.0

    Tire God 2.0 Senior Hellcat Member

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    A 102 SL at 32 psi works.
     

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