Need help with tire pressure for my 35's please

PaulNY

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I just installed 35" X 12.50 X 17" Goodyear Fierce Attitude M/T tires on my '18 JLU Rubicon. So, I think my rig has a curb weight of 4,455lbs but I have added the tailgate reinforcement, and a few other things so I say my curb weight is closer to 4,600lbs give or take. My tires max load (lbs) is 3,000 and max tire pressure (lbs) is 50. I don't understand how to calculate the correct pressure, and a call to Goodyear didn't help as they said "we usually recommend around 8-9 psi lower than max tire pressure". So, can someone please help so that I am not prematurely wearing out my tires? Thanks so much.





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FatBoy01

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Paul, Consider chalk testing. Only problem is you need a dry parking lot, enclosed mall garage, etc. to do the test. You won't find dry in NY today..

Anyway....

-In the morning, before sun beats down on your tires and before you drive the truck-

Instructions:
  1. Fill the tires to a good starting pressure. If you are unsure what pressure to start with then subtract 10 percent from the manufacturer's maximum inflation pressure on the side of the tire. If, for example, the maximum is 40 psi, then 10 percent of 40 is 4. Subtract the 10 percent value from the maximum psi value. So, 40 - 4 = 36. Use 36 psi in your tires. You could also use the following calculation for the starting pressure.
    • Weight of vehicle / (Manufacturers weight rating x 4)) x Manufacturers inflation pressure at weight rating
  2. Drive to a flat area. This can be your driveway or the street in front of your house. Just make sure there aren't a lot of cracks, bumps or pot holes in the ground.
  3. Using chalk, draw a thick, straight line across the width of the tire.
  4. Use Test Variance 1 or Test Variance 2 for the completion of the instructions.

Test Variance 1
  1. Drive the truck forward at least one-full truck length.
  2. Inspect the chalk on the ground. A tire with the proper air pressure should press the chalk line evenly across the ground. This means you'll see the entire chalk line imprinted on the ground. If your tire is over-inflated, you'll only see a small portion of line in the center. If you're tire is under inflated, you'll see only the sides of the lines since the middle of the tire is not making contact with the ground.
  3. Adjust your tire's air pressure according to the chalk test results and try again until you get the chalk line to press evenly across the ground.

Test Variance 2
  1. Drive the truck to the end of the street and back or around the block.
  2. Inspect the chalk on the tire. A tire with the proper air pressure will show the chalk evenly worn across the tire. If your tire is over-inflated, the center of the line will be worn more than the edges. If you're tire is under inflated, the outer edges of the line will be worn more than the center.
  3. Adjust your tire's air pressure according to the chalk test results and try again until you get the chalk line to evenly wear across the tire.

Over-Inflated = More wear on the center of the tire = center of chalk line visible on ground = chalk line worn more in center

Under-Inflated = More wear on the outer edges of the tire = outer edges of chalk line visible on ground = chalk line worn more on outer edges

Chalk-Test.jpg
 
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ckorte13

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Paul, Consider chalk testing. Only problem is you need a dry parking lot, enclosed mall garage, etc. to do the test. You won't find dry in NY today..

Anyway....

-In the morning, before sun beats down on your tires and before you drive the truck-

Instructions:
  1. Fill the tires to a good starting pressure. If you are unsure what pressure to start with then subtract 10 percent from the manufacturer's maximum inflation pressure on the side of the tire. If, for example, the maximum is 40 psi, then 10 percent of 40 is 4. Subtract the 10 percent value from the maximum psi value. So, 40 - 4 = 36. Use 36 psi in your tires. You could also use the following calculation for the starting pressure.
    • Weight of vehicle / (Manufacturers weight rating x 4)) x Manufacturers inflation pressure at weight rating
  2. Drive to a flat area. This can be your driveway or the street in front of your house. Just make sure there aren't a lot of cracks, bumps or pot holes in the ground.
  3. Using chalk, draw a thick, straight line across the width of the tire.
  4. Use Test Variance 1 or Test Variance 2 for the completion of the instructions.

Test Variance 1
  1. Drive the truck forward at least one-full truck length.
  2. Inspect the chalk on the ground. A tire with the proper air pressure should press the chalk line evenly across the ground. This means you'll see the entire chalk line imprinted on the ground. If your tire is over-inflated, you'll only see a small portion of line in the center. If you're tire is under inflated, you'll see only the sides of the lines since the middle of the tire is not making contact with the ground.
  3. Adjust your tire's air pressure according to the chalk test results and try again until you get the chalk line to press evenly across the ground.

Test Variance 2
  1. Drive the truck to the end of the street and back or around the block.
  2. Inspect the chalk on the tire. A tire with the proper air pressure will show the chalk evenly worn across the tire. If your tire is over-inflated, the center of the line will be worn more than the edges. If you're tire is under inflated, the outer edges of the line will be worn more than the center.
  3. Adjust your tire's air pressure according to the chalk test results and try again until you get the chalk line to evenly wear across the tire.

Over-Inflated = More wear on the center of the tire = center of chalk line visible on ground = chalk line worn more in center

Under-Inflated = More wear on the outer edges of the tire = outer edges of chalk line visible on ground = chalk line worn more on outer edges

Chalk-Test.jpg
Wow! Impressive. Thanks for sharing.
 

RubiRob

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If you have the factory wheels, chalk testing will not work. The tires are on the wide side for a 7.5" wheel. about 28-30 should suffice on 35's and up.
 

BillyHW

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Should I be close to that in my C load rated 315/70/17 BFG KO2s?
So I can't find any LT Metric load tables that go below 35 PSI. Only the Flotation LT tables seem to go down to 25 PSI.

I'm not sure how valid it is to interpolate to the left of the 35 PSI point. (It's easier and more valid if you are interpolating between 2 data points.) I also don't know if you should go lower than 35 PSI on LT Metric tires because they don't have those data points in the tables, and I don't know why that is. Maybe you're not supposed to but I don't know.

Here is the load curve for your size tire. I'll let you decide where you think the blue curve would hit the orange line.

CaptureTirePressure2.JPG


Also please make sure you don't go over the Max Load and Max Pressure that is written on the side of your tire. As it's a load C2 tire, you are actually cut off at a maximum of 35 PSI/2535 lbs on the tables, but you might be able to go higher depending on what's written on the sidewall.

That seems like an odd tire size, because according to the tables, 35 PSI is the *only* possible tire pressure.
 
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RubiRob

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So I can't find any LT Metric load tables that go below 35 PSI. Only the Flotation LT tables seem to go down to 25 PSI.

I'm not sure how valid it is to interpolate to the left of the 35 PSI point. (It's easier and more valid if you are interpolating between 2 data points.) I also don't know if you should go lower than 35 PSI on LT Metric tires because they don't have those data points in the tables, and I don't know why that is. Maybe you're not supposed to but I don't know.

Here is the load curve for your size tire. I'll let you decide where you think the blue curve would hit the orange line.

CaptureTirePressure2.JPG


Also please make sure you don't go over the Max Load and Max Pressure that is written on the side of your tire. As it's a load C2 tire, you are actually cut off at a maximum of 35 PSI/2535 lbs on the tables, but you might be able to go higher depending on what's written on the sidewall.

That seems like an odd tire size, because according to the tables, 35 PSI is the *only* possible tire pressure.
Yea its the only way to gear near 35" tires without having to go to a E load rating. Fairly popular tire size according to how quickly tire rack has been selling them. I have them at 35 right now, however discount tire air'ed them up to 40. Thanks for that info!
 

BillyHW

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Yea its the only way to gear near 35" tires without having to go to a E load rating. Fairly popular tire size according to how quickly tire rack has been selling them. I have them at 35 right now, however discount tire air'ed them up to 40. Thanks for that info!
Just out of curiosity, what are the max load and max psi listed on the sidewall?
 

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