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Discussion Starter · #1 ·
I have a 2004 F250 6.0 crew cab lariat. The stock tire size is 265/75/16 and the door jamb says psi is 55 front, 70 rear. I have larger tires now which are 285/65/18. Should I keep the psi the same? And if I tow, what should I have psi set to?
 

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PSI when cold is what the question really is here. I try to run my tires cold at 65PSI Front/70PSI rear. Tires will heat up when rolling and you want to be within 10% of the cold PSI setting. So set the PSI when cold, then after driving a bit check the PSI again.
If you are going to tow heavy the rears need to be close to the max PSI as listed on the tires. For the fronts it will be subjective to the ride.

What's the listed PSI for the 285/65/18's??
 

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Your door jam sticker is for max load.
If you really want to be exact, pull the tire manufactures site up and find the tire psi chart for weight on axle or tire. This method requires a trip to the scale for empty or loaded conditions to be exact.
 
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I have a 2004 F250 6.0 crew cab lariat. The stock tire size is 265/75/16 and the door jamb says psi is 55 front, 70 rear. I have larger tires now which are 285/65/18. Should I keep the psi the same? And if I tow, what should I have psi set to?
You can't really go by the sidewall, door sticker or charts you see on the internet. Its takes a combination of test to TRULY find the right PSI based on needs and expereince.

the chalk method is great for treadwear but not always accurate for towing.
the charts are GENERAL GUIDELINES and should not be taken as 100% fool proof.
max loads are generally taken out of context. My bfg at ko2s are 285/75r16 with a max psi of 80 psi for 3750lb each. The rear of my truck does not even get close to 7500lbs empty and 99% sure i've never done that towing as I don't use a truck bed camper anymore. Get your truck empty (generally speaking) total, and then front/rear axel and start around 50psi and go up or down from there. Because it will vary by the tire make, truck, and when you tow.

For example empty i use 55psi front and 50 rear. give or take a few psi. But even with my 33or 36ft gooseneck (I always forget) I have never needed 80psi. Heck I drove around on 50 psi for an entire summer and never had a sway, tire roll or odd issue lol. I put the rear at 65psi most times for my normal loads. You just start somewhere say 40-50psi for most empty trucks and test it out for treadlife, sidewall stiffness, and general emtpy/loaded weights and go from there.
 

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Discussion Starter · #5 ·
So here’s what I ended up doing…I weighed the truck without trailer. Then I added the weights of my cargo and Tongue Weight. Then I calculated, based on the max load capacity/psi and found how many psi per pound. Then I multiplied the weight by that formula. Based on that, with my tires and trailer, the fronts should be 55 psi and the rear at 60 psi.
 

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Without knowing the exact tire and recommended PSI on the side of the tire, that sounds WAY too low for that size tire.
 

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Discussion Starter · #10 ·
Okay. So I’m gonna throw this out there to see if I’m not calculating this correctly, but here’s my PSI math.

Stock tires were 265/75/16 and the door sticker says 55 front, 70 rear. And that’s them thinking you have a full payload.

My new tires/wheels are 285/65/18 with max PSI of 80 at 3640lbs. Times that by 2 for the 2 rear tires, and it comes out to 1psi per 91lbs.

I weighed my truck with a full tank, no trailer., and the front is 4220lbs, rear 3040lbs. My trailer is a bumper pull, and the tounge weight will never be close to 800lbs. So with my weight distribution hitch and passengers, I figure I’m adding about 700 lbs to the rear axle and maybe 150 to the front.

That means a rear weight of about 3740 and a front of about 4400. Per my math above of 1 psi per 91lbs, that means with my trailer hooked up and weight distribution on, the front tires would be about 50 psi, and the rears at 41psi. I added some psi to be safe and came to the 55 front and 60 rear calculation. Am I wrong?🤔

The weight of the actual trailer loaded is about 7k. But I think the only thing that factors into the tire psi is the tounge weight?
 

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that guy really likes his chalk


Park on level ground and aim front tires straight ahead.
Draw a line completely across the tread.

Pull ahead 2-3 tire revolutions or until the chalk starts wearing off.

If the chalk mark is worn off equally across the width of the tread, the inflation pressure is correct.



If the chalk mark is worn off more in the center than at the edges, as shown here, the tire is over inflated.

If the chalk mark is worn off more at the edges than in the center, as shown here, the tire is under inflated.

courtesy of Proper Tire Inflation
 

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The problem with the math method is you have no idea about the tire construction stiffness in the wall or in the tread, especially the difference from one tire model or brand to another.

The caulk method is a good DIY way. The pro way and WAYYYYYYY more expensive is to use FujiFilm Prescale. It's how I've seen it done while at the Transportation Research Center in OH, where NHTSA and many other companies test. I've also seen it done at a Smither's test track in Texas where Michelin and BFG were testing.

While "I am a need to know every aspect kind of guy" (ZMANN reference), I use chalk, too.

Font Line Slope Automotive lighting Circle
 

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Discussion Starter · #19 ·
The problem with the math method is you have no idea about the tire construction stiffness in the wall or in the tread, especially the difference from one tire model or brand to another.

The caulk method is a good DIY way. The pro way and WAYYYYYYY more expensive is to use FujiFilm Prescale. It's how I've seen it done while at the Transportation Research Center in OH, where NHTSA and many other companies test. I've also seen it done at a Smither's test track in Texas where Michelin and BFG were testing.

While "I am a need to know every aspect kind of guy" (ZMANN reference), I use chalk, too.

View attachment 785436
This is great info. I will definitely be doing the chalk method. I really appreciate all your guys insight on this stuff. I always try to be safe and informed with my vehicles, especially when towing, so having this info is awesome. I kinda wondered if my math method was legit, but sounds like this chaulk method will give me a more definitive answer. Thanks again!!!
 
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