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tire pressures- anyone use this formula?

fordtech

NAXJA Forum User
Location
Amherst NY
snip from Jeepforum:
...For an example I’ll use a tire rated at 2270 lbs @ 50 psi and a vehicle that weighs 4850 lbs.

Step 1 - Multiply the max load times 4. This is the maximum weight your four tires can safely carry.
2270 lbs * 4 = 9080 lbs

Step 2 - Divide the weight of the vehicle (placard on driver’s door jam) by the number you got in the first step. This is the percent of the max load you are actually carrying on the tires.
4850/9080 = 53%

Step 3 - Multiply the psi stated on the tire for max load by the percentage you got in step 2. This will give you an optimal pressure for you tire on the road.
50 psi * 53% = 27 psi


I used this with my xj at 3,386 LB (internet xj curb weight, I'll use my door sticker later), tires rated at 2270@50 PSI (off Coopers website), and got 18.5 PSI, which seems low. I also have used Gojeeps method (pasted below) and have been in the 26-28 PSI range. I haven't gotten a chance to do the chalk or crayon method due to the wet weather we have had in NY lately...

Interested to see what others have tried, and if either of these methods resulted in positive results.

Snip from Gojeeps site:
...you take front and rear pressures cold, and then again immediately after at least 20 minutes at highway speeds or more and there should be a 4 psi increase. If there is more than 4 psi, than you had too little to start with and if there is less than 4 psi, than you had too much.
 
fordtech said:
and got 18.5 PSI, which seems low.
NEVER go below the min pressure recommended on the placard on the driver's door. The engineers came up with that as the safe pressure for your vehicle in how its designed to be used.

Too low of pressure results in lots of bending flexing of the sidewall (from the buldge) which increases friction/heat and degrades the tire and can even cause it too fail. That was key factor behind the whole Ford Explorer/Firestone debacle, they recommended too low of a pressure for the vehicle (to get a good ride), instead of redesigning the suspension, used a cheap Firestone tire that had problems with its tread compound and tread coming off if overheated, on a vehicle that had a ridiculously low rating for cargo. So typical ignorant owners, didn't check tire pressure, letting it get even lower, overloaded their vehicles and took long trips in the summer, boom tire blew out.

I never seen that formula before, it interesting, but what I do is too monitor my tire wear, and adjust tire pressure accordingly.

I.E.
Even Tire wear, good pressure
Outside edge wearing more, increase tire pressure
Inside tread wearing more, decrease tire pressure.

If your alignment is good, the only reason the tire would NOT wear properly is that improper air pressure for your typical vehicle conditions and driving style, is causing the tread to deflect too much and not keep a proper contact patch on the pavement.

Optimum Tire Pressure is so dependent on so many different factors, and which is your priority to get the desired performance. So, getting the ideal tire pressure is always going to be a compromise balancing one factor off another. Determining best tire pressure off the wear of the tires, is going to give you the best compromise, because it is most effectively averaging out the factors in the use of the tire to give you best average contact patch, resulting in the best average wear on the tires. I typically have to use a higher pressure on the front tires than the rear, that also is very typical, and does give better handling/traction characteristics, especially in RWD vehicles.

Your limits in experimenting with the tire pressure always has to be;
MIN - Tire Pressure Recommended by the manufacturer on the placard on the driver's side door.

MAX - Max Tire Pressure Recommended by the tire manufacturer on the side of the tire.


Remember, the tire often list a higher pressure that is permissible temporarily to just seat the bead during tire mounting, that is NOT an permissible pressure to use during driving.

My Mini-Van recommends 36 PSI from Chrysler, but there are lots of tires that are marketed for my Mini-Van that list 35 PSI as MAX pressure on the side of the tire. So, many vehicle today, if you don't do your research, could end up with having to exceed one of the limits to use it.
 
Last edited:
WOW.

To start, what are you baseing your coment regarding never going below factory specs? That spec is set with a stock size and style tire. I have LT 31x10.50x15 Cooper STT's, which granted I didn't mention. Yes, 18 psi is way too low.

I'll agree on the firestone statement. I changed out alot of tires during that recall, and know why.

As far as monitoring wear, its my opinion that once I start seeing uneven wear, you really can't reverse that wear. So basically you'll have that wear for the life of the tire, not a good option really. I am very aware of how to read a tire's wear.

So you are saying that the 50 PSI listed on my tires is the max bead seating pressure? I would argue to say that is the recommended pressure of the tire with the stated load (in this case 2270 lbs), but it is also the max pressure, so I agree with you I guess.

I appreciate your post, and welcome your comments, but it was a little more than I wanted to get into!! :)

As an update to my original post... the door jamb says 4900 GVW, so I figure I'm usually not loaded to full capacity, so lets say 4500 lbs (still high probably). That works out to about 25 psi, closer to the gojeep method.
 
in my experience with rigs and tires (use to work for a couple shops) its usually about 20-30 psi most rigs are happiest. I honestly never use less than 20psi on the street i know people who run 35+psi and wear out the center of thier tires I know people who run 20 psi and wear out the outer edge and all swear its the tires fault so I suggest starting in a middle ground spot at 25 and make small adjustments till you get the desired result. every rig is differnt
 
I run my BFG AT/KO's at around 35-40lbs, this made the ride a tad harder but stopped the uneven wear of the lugs. Now they last me close to 80,000mi before a new set. At 30lbs the front lug wear is very evident and I rotate them so that they even out.
Will be doing 4 new ones soon as my wheels get back from being polished and re-clear coated.
 
RichP said:
I run my BFG AT/KO's at around 35-40lbs, this made the ride a tad harder but stopped the uneven wear of the lugs. Now they last me close to 80,000mi before a new set. At 30lbs the front lug wear is very evident and I rotate them so that they even out.
Will be doing 4 new ones soon as my wheels get back from being polished and re-clear coated.

seems high, but I believe you!

My last set of 31's wore quicker in the middle at 28 psi. I ended up with 6 32nds in the middle, 10 32nds on the outside. I called Mickey Thompson for advice when I first got them and the fellow I was speaking to said it was due to the stock steel jeep wheels being only 7" wide..... makes sense, but seemed like an excuse at the time.

I really don't want to have these tires wear uneven if at all possible!

Thanks for all the input.
 
I run my MT 33's at 32psi

The chalk trick works wonders :D
 
fordtech said:
To start, what are you baseing your coment regarding never going below factory specs? That spec is set with a stock size and style tire. I have LT 31x10.50x15 Cooper STT's, which granted I didn't mention. Yes, 18 psi is way too low...

As far as monitoring wear, its my opinion that once I start seeing uneven wear, you really can't reverse that wear. So basically you'll have that wear for the life of the tire, not a good option really. I am very aware of how to read a tire's wear.

I appreciate your post, and welcome your comments, but it was a little more than I wanted to get into!! :)

As an update to my original post... the door jamb says 4900 GVW, so I figure I'm usually not loaded to full capacity, so lets say 4500 lbs (still high probably). That works out to about 25 psi, closer to the gojeep method.
Actually, after I made that post, I thought about some of the big aftermarket tires people put on their vehicles. I can see a drastically different tire, using other than recommended pressure, especially if much larger. As well, off road, under certain conditions while being kept to lower speeds, I can definitely see using a lower pressure safely.

Its highway speeds that I would worry.

Max pressure on the side of the tire, we agree, I point that out, because I've seen post from people that found the fine print on the tire that stated a higher limit for bead seat while mounting the tire and then professed that it was OK to run the tires on the road at that higher pressure. No, use the max pressure stated on the side of the tire, that corresponds to a max load, like you stated.
 
Rick Anderson said:
Actually, after I made that post, I thought about some of the big aftermarket tires people put on their vehicles. I can see a drastically different tire, using other than recommended pressure, especially if much larger. As well, off road, under certain conditions while being kept to lower speeds, I can definitely see using a lower pressure safely.

Its highway speeds that I would worry.

Usually the large sized tires are LT vs. P rated (Light truck vs. Passenger). The LT's have heavier, and usually stiffer sidewall due to being designed to handle more weight and abuse. So you don't need as much air pressure to hold the weight up as you would with a lighter load rated tire.

I agree, that too low a pressure can be dangerous. My point is that vehicle XYZ might require 33 psi in a P225/75/15, but 25 psi in a LT 33x10.50x15 to hold it up.
 
The less sidewall flex, the less heat and problem. I'd access the bulge of the tire sidewall when using less air pressure, the faster you'll be driving the less bulge is acceptable. I can see with some of the light truck tires getting down to 25 PSI and still having the same amount of bulge as passenger tires at 33 PSI.

Still, I'd worry a bit if travel at higher highway speeds like 70mph at less than manufacturer recommendation, I'd pull over when first trying the lower pressure and feel the sidewalls and treads of the tire for heat, make sure it wasn't getting too hot.
 
lawsoncl said:
Don't forget that the wheel width makes a difference too. For example, you might be running 31's on a 7" rim or an 8" rim.


Gotta wonder what would happen with an 8" ot 9" wide rim.

I'm suprised so many people would run such low tire pressure, I even ran 35/30 psi on the front/rear of my Turbo K-Cars, let alone on the P235/75/R15 Wranglers I'm running now. Dad's experiance with running his P235's@35psi have been pretty much identical to Rich's whenit comes to tire wear too.
 
Rick Anderson said:
The less sidewall flex, the less heat and problem. I'd access the bulge of the tire sidewall when using less air pressure, the faster you'll be driving the less bulge is acceptable. I can see with some of the light truck tires getting down to 25 PSI and still having the same amount of bulge as passenger tires at 33 PSI.

Still, I'd worry a bit if travel at higher highway speeds like 70mph at less than manufacturer recommendation, I'd pull over when first trying the lower pressure and feel the sidewalls and treads of the tire for heat, make sure it wasn't getting too hot.

This is the theory behind Gojeeps write up.... right now my 31's are at 28 cold, and after driving get to about 31 psi. they are warm to the touch.

I know what your saying but I still maintain that a larger LT needs to run less PSI then a P rated tire, and that LT will function the same at that lower pressure as that P does at factory spec.
 
I run larger 31's and have run 30PSI. This seem to have a more even wear and gives me a smoother ride. On the trail I drop to 20-25 depending on trail.
 
I just called BF Goodrich and they use a chart based on vehicle weight and stock tire size and stock tire pressure to get you the tire pressure you'd need for upgraded tire size.

They told me to use 35 PSI with my BF Goodrich AT/KO's (30x9.50).
 
Blaine how do you contact B F Goodrich. Do you have a phone number or an e-mail or what. I could not find a way to contact them.
 
United States:
Call 1.877.788.8899 during normal business hours
(Monday-Friday: 8:30 am to 8:00 pm, EST; Saturday: 8:30 am to 4:00 pm, EST)


Just get the stock tire size and pressure from your door and give it to them......and let them know what kind of tire and size you went up to.
 
Blaine B. said:
I just called BF Goodrich and they use a chart based on vehicle weight and stock tire size and stock tire pressure to get you the tire pressure you'd need for upgraded tire size.

They told me to use 35 PSI with my BF Goodrich AT/KO's (30x9.50).

right in line with Rich...

As soon as it drys up around here I will take it to a parking lot and do the chalk test and see what it comes up with. I will be suprised to see that if is more than 30 psi.

Again thanks for all the replies, its brought up some interesting views, facts, and experiences.
 
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