How to calculate street pressure for larger tires.

Stumpalump

Expedition Leader
https://www.tacomaworld.com/tirecalc?tires=235-75r15-255-70r15

Using the above link enter the tire size found on your door sticker and your new tire size.
Add the width percentage increase and the side wall height percentage increase. Take that total percentage and subtract it from the pressure recomended on your door sticker. That's your new pressure.

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So in my case above The stock tire was 235/75-15 and new larger tire is 255/70-15.
It's 8.5% wider and has 1.3% increase in sidewall height or 9.8% that's a 9.8% increase in air volume.

32 psi is recomended on the door panel so I multiply it by .098 and that equals 3.136.
Now I subtract the 3.136 from the original 32 and get my new pressure of 28.864 psi. lets say 29 pounds.

Does this math using percentage of volume vs pressure add up?

https://youtu.be/GBtbirNyzMA
 
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eggman918

Adventurer
When i went to the S/T Maxx's on my truck i got axle weights front and rear and Emailed Cooper and they got back to be within 24 hours with the inflation numbers for my application, excelente customer support from them.
 

verdesard0g

Search and Rescue first responder
I just use a load table and the weight of the front and rear axles to determine correct tire pressure for any vehicle and/ or tire size. No calculations needed.
 

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dumprat

Adventurer
Chalk works. Rub it on the face of the tire. Drive forward on pavement a few yards. Drop pressure until the chalk wears off evenly.

You can do the same method with a digital temp gun.
 

javajoe79

Fabricator
Any given tire has a given spring rate at a given pressure. The tire manufacturer should know those numbers. I would try to match spring rates between tires or tailor the pressure to suit your application. Just don't run over the max on the sidewall or too low. Too low and heat buildup at speed and loss of MPG will be a problem
 

Stumpalump

Expedition Leader
I could not find a load table for General Grabber AT2 so I just used a similar Toyo table. It came to the same pressure. I went to 32 on the table to match the factory recomendation and found the weight for the old size. I matched that weight to the new size and came out with a little less than 29.
 

IdaSHO

IDACAMPER
Any given tire has a given spring rate at a given pressure. The tire manufacturer should know those numbers.
Yes, they SHOULD

Assuming they have such data, very few share it.


I have always used TOYO load charts as a starting point, then use chalk.
 

rayra

Expedition Leader
No I don't think it adds up, it's not a linear relationship between volume and pressure, is it? Besides which, you're slavishly trying to match the original tire 'street pressure' to what, mimic or maintain a factory performance / handling characteristics? after you've changed the brand, type of tire, vehicle center of gravity, size of the contact patch(es), rolling forces, braking traction etc etc?

Doesn't seem a worthwhile pursuit to me. Too many other factors at play for it to even matter. It's a street truck, not a multi-million dollar race vehicle or a space shot to intercept an asteroid.
 

Ozrockrat

Expedition Leader
Heat or chalk (or a pressure table but I haven't seen one of those in 20 years). On big tires they give you a temp chart which you use to calculate rate of temp rise over cold temp. I doubt they have that for these little tires but the same concept applies. If the temp is rising too quickly you are flexing the carcass too much so increase the pressure. Chalk coverage will give you a full/even contact patch but some tires need to be run with a little higher pressure than that to stop them cupping quickly.

OK tried to attach the file but it is too big so hit me with a PM with your email address and I will send you a copy of the specs for my tires. This will show you the information the tire guys have but getting them to release it to the public is a different matter.
 

PJorgen

Observer
I just use a load table and the weight of the front and rear axles to determine correct tire pressure for any vehicle and/ or tire size. No calculations needed.
^^^ This is the way to do it. I would add that you should use the load index (not load range) found on the sidewall of the tire, as well as the tire size. Use that information with the vehicle's weight to look up the correct pressure on a load table. This information is NOT brand-specific. Load index is an independent value based on the tire's construction.
 

dumprat

Adventurer
So based on that chart a load range E tire on a Tacoma should be able to run at less than 24 psi. i am sure that would make for a squishy ride but wouldn't do the tires any good.

Use chalk or a temp gun. If the tire is under inflated and run at high speed they can separate. Same goes for airing down to single digits to run logging roads with lots of weight on.
 

downhill

Adventurer
The ONLY reliable method I have found is chalk. The heat reading should work, but heat travels and the measurements are tricky. My tires wear very evenly across their life.
 

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Happy Joe

Apprentice Geezer
The sidewall pressure is for maximum load and not really/universally applicable to best pressure for optimum life/mileage at lighter loading than maximum.
Take your best guess with a pressure table or by factoring the actual vehicle tire loading then do a chalk test to confirm the results and adjust the pressure to give the best chalk test; max pressure will normally wear the center of the tread out resulting in the tires being unnecessarily worn and replaced sooner than they should (IMO).
..Obviously when you load the vehicle heavily you need to increase the tire pressure (repeat the above steps for the new load).
...As a safety factor; check the tire temperature at each highway refuel stop and increase the pressure if the tires are running hot (do not exceed max pressure).
Enjoy!
 

downhill

Adventurer
A couple of points worth mentioning I think.

Tire construction plays a significant role in this whole determination. If you have three identical size tires with the same load rating, you will see differences in the sidewall stiffness. The difference between a Toyo, a Cooper SST, and a Mastercraft are very apparent. The Toyo is so stiff that one could image not needing air at all in a light truck, The Mastercraft is soft and very comfortable on the road. They are 3 ply and 2 ply sidewall respectively. The Cooper is 3 ply, but lands between the other two tires in stiffness.

Boyle's law would apply perfectly if all tires were constructed the same. Because they are not, Boyle's law must be adjusted a few pounds. A concept worth grasping is that a given weight of truck must ride on a given volume of air. The tire's purpose is to contain that volume, either by allowing for high pressures, or just being a larger volume tire. If you have two identical trucks, running on similar constructed tires, with one truck riding on huge balloon tires, and one on skinny road tires,.....both tire sets would contain the same volume of air (corrected at some standard elevation and temperature). The air doesn't care what size tire you squeeze it into, as long as the correct volume for the vehicle weight is there. In other terms, a 10 cubic foot volume tire might run at 50 psi, while a 20 cubic foot volume tire on the same truck would run at 25psi. Both contain the same volume of air.

I bring this up because I have had many "discussions" with people who believe that a vehicle must run on a certain pressure of air. Not so. I have also had "discussions" with people who believe that because a tire has a higher maximum pressure rating, that it MUST run at a higher pressure. Also not true. A load range C tire and a load range E tire of the same size, on the same vehicle, would require the same pressure. In fact, the E tire would actually need a few pounds LESS, because of the increased sidewall stiffness.
 
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