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Tire Pressure in Aftermarket Tires

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k5zx

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I bought 48 inch thornbirds for my 75 k5 and filled them to max because I was told that they needed to be filled to max pressure unlike stock tires that go byu the sticker specs on the inside of the door sticker.

Now I've heard that there is a formula to figure out how much aire should go in the tires taking into account diameter, speed, weight of truck, etc

Can anyone tell me how much air should go in my tires or do I need to calculate it somehow?
 
there is a formula. I dont know it, it was in 4 wheeler magazine. but I do know that it will be way lower pressure than your factory tires are supposed to be due to more contact area
 
You have to find out or estimate the weight of your truck. Then you find the max pressure in your sidewall for that tire. Divide your weight by 4 to get your by tire Cappacity and times that percentage by the max tire Cappacity. So if the truck weighs say 5000 pounds and you max weight per tire is 3500, you divide that to find percentage difference and times that by max tire pressure. That will give approximate air pressure to put your tires to. Hope that makes sence.
 
or the easy way alot of people use ( i havent yet) is the chalk test. PUT a very heavy chalk line all the way across the tread and drive straight in a parking lot for a bit. then check the wear pattern in the chalk to see what your contact patch looks like. to much wear on the center- take air out,,,,,, too much on the outside edges- add air. supposedly easy. never used it
 
Those are the two methods I've seen. Wasn't sure his truck on 48s was legal on the road so was just giving him a good baseline to get it close in the driveway. Ive hadn't had a problem with weird tire wear using my method. The chalk one seams like more work so I've never used it myself either.
 
They can be AWESOME if you get out the 4" grinder and remove half a lug in a few key areas of the pattern. I still to this day believe that they made them for the guys with groovers:haha:
 
jekquistk5 said:
There lies your problem. Thorbirds are the worst tire, unless you like going to the mall.
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Actually they are worthless on street as well. Unless you like an unbalencable tire...

I've seen some new swampers come with 10 once patch inside on one side and still couldn't balence it?
 
thanks for the responses. Especially the relevant ones. I spent hours trying to find the formula.

I also have an s197 mustang I bought wide tires for but I assumed the difference wasnt enough to make a difference like the thornbirds were.

I bought the thornbirds when I was new to offroading since then Ive read alot of people stating they are no good.
 
chulisohombre said:
You have to find out or estimate the weight of your truck. Then you find the max pressure in your sidewall for that tire. Divide your weight by 4 to get your by tire Cappacity and times that percentage by the max tire Cappacity. So if the truck weighs say 5000 pounds and you max weight per tire is 3500, you divide that to find percentage difference and times that by max tire pressure. That will give approximate air pressure to put your tires to. Hope that makes sence.
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So I multiply the pounds by 1000 like above? 35 lbs psi times 1000 =3500

I worked through the folrmula as I understood it and wasnt sure it made sense

5000 divided by 3500 is 1.4285
multiplied by max air pressure (35) is 50.

Is that right by your example?
 
Chalk works the best.

I assume they are 38" tall thornbirds not 48" cause those don't exist?

With 38s you should be nowhere near max pressure, which actually on a big bias ply tire like that should be closer to 35 psi max.

What I have heard is you should have a 4 psi rise after driving around for a bit ( probably 30 minutes at least) that indicates proper tire pressure.

More than 4 psi and you have too much pressure, less than 4 psi and you have to little.

I bet your ideal pressure will be closer to 20 -25 lbs
 
thay dont make a 48" thorn bird .

and if truck is 5000 total thats fine. but you need weight per axle front / rear as thay wil not be the same.

then ( example ) take tire that shows 3600 lbs @ 50 psi max. divide this out to 72 lbs per psi of air it can support.

then if your front end is 3400 lbs alone on the 2 tires divide this by 2 = 1700 lbs per side. then take this 1700 and divide by the 72 lbs per psi and you get 23.611 psi per tire. so just round that to 25 psi to be safe. and do them all the same psi for easy maint and roation safe if you forget to swap psi around.

this is all example #'s and road test needed to confirm safe road handling and not over heating tires from to low a air pressure.

also for example as listed above the tire chalk test works great also. i have done this my self and had super results.

i found my old 38x12.50 tsl swampers on my 7000lbs 1ton truck ran best at 25 psi each .
 
Spray paint works even better then chalk. After the formula given you are close enough for paint or chaulk.
 
back story and tire size

Ive learned more about big tire pressure in this thread than I have in hours of search engine research (including on this site). Someone should write an article for it here or at least a permanent post.

Anyway thanks for all the guidance.

The background is I got this truck in about 1994 a couple years after i got out of the army for 3500. It was the coolest truck I had ever seen, and normally I like Fords. I had looked at Broncos, but I couldnt find one that was full convertible that was as big as the blazer (the body I mean).

I ran out of money, the 33x12.50s were stolen off it, etc. I finished my degree, became a teacher, got married, all the time never driving it just starting the engine on it a couple times a month.

Just before I got married I got the thornbirds. After I got married I got a job 8 hours away in another part of the state that has no mud, no rocks, no nothing really, and I didnt even know if the truck would make it there. The last time I drove it, a fire started under the engine (the fuel filter line had rotted).

So I drove it 10 miles to my father in laws house and its sat there since 2007 til now. I just moved back to the ozarks part of the state with lakes, mud, rain, rocks, trees, people, etc (hence my absence from this site from 2007 until this thread last night)

Now I live a little over an hour (in a normal car, it will take me half a day in the blazer if Im lucky) from the father in laws house. And am about to bring the truck home finally to work on it and get it ready to give to my son when he's 16. My son just turned one in September so I should have enough time.

That's why I thought they were 48 inch. I could have sworn thats what they were. Did they make that size in 2007? 44? I could have sworn they were 48. Thats why I got the size wrong. I have hardly seen the truck in years.

Any advice for the trip home other than making sure the lines arent dry rotted? I became an army (reserves) mechanic last year but that teach much unless one gets deployed and gets to do it full time for a year.

I wonder if the spray paint will rub off enough on asphalt as opposed to flat concrete for the paint test
 
k5zx said:
I wonder if the spray paint will rub off enough on asphalt as opposed to flat concrete for the paint test
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Yes and it takes a little longer so you can drive 1/4 mile and see good results. Should wear even. If it wears center to much air, outside to little but that is usually not the issue.

All depends on wheel size as well. 12.5 wide tires on 10 inch rims is easier then 8 inch rims.

The vehicle has been parked since 2007 and your going to get it and drive it home? :haha:

Unless I missed the part of working on it to make it run, you got a lot of work to do? Like five year old gas is your biggest issue... battery, mice nests made out of your wiring etc... who knows?
 
With the scenario above at 35 psi being max pressure you would want 20 psi per tire then check to see contact pattern. I set mine at 25 in front for the heavier engine weight and 20 on rear tires. As big ad you say your tires are you have to really look at the max pressure on the sidewall to know for sure. I think my tires max pressure is 50 lb at Max load 3500 per tire. I found this method like what I use. It gets you very close to what you need to run your pressure at:...


There is a math process that will do the same thing for you.
On the sidewall of the tire you will find the max load/pressure of the tire.

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

The chalk test and the math approach will usually give you very close answers.
 
The split pressure is a good idea, I've done that for years. Depending on rig 2 to 5 pounds lower in rear.

I found this while aired down and drove onto pavement to get fuel, didn't feel right and could not get it out of 4x4, forgot till on pavement. Had to back up five feet to get transfer case out of 4lo. While at pump I looked at tires and fronts were flater then rear, but all same pressure and it hit me! Front weights more.

I'm old and figured this out around 1982-3 before the internet! :haha:
 
k5zx said:
And am about to bring the truck home finally to work on it and get it ready to give to my son when he's 16. My son just turned one in September so I should have enough time.
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Really don't have much to add except I like the background story :thumb:
 
Yeah my son and I had fun with cars, trucks and off roading. Well until he was about 13 and then I never got to drive off road anymore... :doah:
 
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