Check my Math - Tire Pressure by weight..

I am loaded for bear and headed west, and I am sitting heavy. I found a PSI formula on the internets that says:

Tire Weight / Tire Pressure = Load Capacity Pounds per PSI

So My Tires currently mounted are 3970 @ 80 PSI.

My weight is about 6500. Lets just say 3500 in back, 3000 up front. ( I am headed to the scale later today to verify)

If I do the calculation I am looking at 49.625 pounds per PSI. If my rear end is at 3500 pounds, then I am looking at 70PSI, but 3500 lbs is both rear tires. So do I divide this in half? thus 35PSI?

Currently I am sitting at 40PSI but was thinking I should go up a little, but now I am not sure.. On my F150 I ran my E rated Tires at 50 PSI all the time. I am currently heavier than the F150.

Help a brother out..

 

I usually just do the chalk test.

 

I knew I was going to hear that .. lol. I was hoping for something a little more scientific and less time consuming..

 

So I guess I know what I am doing at lunch..

 

If you're running heavy for a great distance, pump up to the high limit on the tire sidewall.

 

Yeah I am going 1800 mile each way, plus another 1200 or so on and off pavement in the Wyoming Montana area which I will be up and down on pressure.. Mostly trying to figure the best pressure for the trip out and back.

 

I think the chalk method might be a little more accurate in this case than a general formula. Tire construction will play a huge roll as well. A tire that has a stiffer sidewall will flex less with less air. Sidewall flex dictates how much tread patch is making contact with the road, and how much heat is generated in the tire.

 

Fwiw when I called Toyo about recommended tire pressure for my truck they said the 285/75r16 isn’t designed to run with an air volume below 35psi even though the weight requirement of the truck would suggest an air pressure lower than that. For instance my 285’s at 35psi are rated for 2130lb each.

 

Did they give you a pressure based on your truck weight?

 

that's the corporate answer, actually if you call any tire company they will tell you the same thing, 35 psi is the lowest you can go.

 

I gave them the weight of the truck as I had put it on a set of scales. Just before switching wheels and tires. I haven’t put it back on since but I’ve probably added 100-200lbs as I added a blower, set of sliders but lost 40lbs in wheel and tire weight. I typically run 32 psi front and rear and then when I’m towing our camper I bump it to 35f and 40r and it seems to drive best at those pressures.

 

So my chalk test seemed to show I was in pretty good shape at 40, but I think I will bump it up a little for the ride out..

 

Keep it at 35-ish as your fancy math shows you should. Chalk it there to get the warm fuzzies. At least bring the gauge on the drive to drop it down when you get tired of your teeth chattering down the highway. LOL

Stock PSI is what 28?

 

This " pigheaded Dutch selfdeclared tyrepressure-specialist " has the calculation for you.

In fact the lineair calculation you found is not that bad. The official calculation, I once got hold of, and went running with gives slightly lower pressure.
But more important is thst the weights are right, and that is mostly yudged to low.
So its wise of you, you planned weighing, but then do it in the loading you go on trip with, also the persons.

And needed pressure is also dependent on speed used, max speed, you wont go over for even a minute.

That is why I add in my made spreadsheets a reserve of 10% , and calculate pressure for 99mph/160kmph, most tires are calculated in maxload for that speed. Then you have maximum reserve, with still acceptable comfort and gripp, so I determined in time.

And even if weighed, this reserve is needed for next.
1pressure-loss in time
2unequall weight R/L on the axle.
3.misreading and inacuracy of weight and pressure.
4 to 10 etc etc.

So you calculate with max reserve, but can end up with yust enaugh to prefent overheating of one tire, wich is the goal of pressure determination.
This overheating is only allowed ZERO times in livetime of tire, and can cource blowing tire or treath-separation , mayby only after 3 years.

 

Remember that your pressures increase as the tire gets hot. Personally, I do the chalk test when the tires are hot. I'm sure some will debate this. But if you think about it, you're driving around on hot temp, not cold. So if you do the chalk test when it's cold, once it heats up, you're off the mark. I take another reading the following morning to get both temps as extra reference but I always air up targeting the hot reading....change my mind.

 

I am surprised they said that high a psi. My truck came with the Goodyear 265/70r16 and the door sticker said 32 psi. I have bfg 285/75r16 e now. When I first got them I ran 32 which read 35 hot. I had bad vibrations in highway. Did the chalk test and ended at 29/30 cold, 32 hot, front and rear. No more vibrations

 

A stiffer tire ( fi E-load AT 80 psi), needs higher pressure for the same load then a less stif tire( fi P-tire standard load AT 35 psi).

That is why the chalc test ended up in higher pressure. The car advice of 32psi was for the OEM P- tire.

Pressure advice is to give the tire a warm pressure- so deflection, so heatproduction a cycle , that wont give a temp of rubber of tire , above the critical temp, where it hardens and crackes. This may only happen ZERO times in a tires live.

But warm pressure is inacurate to determine, because , how fast did you drive, and how long after stopping did you measure, and external factors influence the temp of air in tire too, like sunshine, severe braking, high Ambiënt temp.

Cold pressure is simpler, when temp in tire i= temp outside tire. But then even advice is for 70degrF. , so if its 90 degr F , pressure is higher. But then it needs lesser deflection the higher pressure it gives, because cooling down of rubber of tire is also worse, because of smaller temp-differences.

Then the chalc test.
A radial tire stays over a large range of deflection/Pressure side to side on the ground, so chalc wears even.
So it can be that chalc wears even, but still at high speed, the rubber goes over a critical temp.

If doing chalc test, begin at to high pressure, and work down until chalc dont wear in the middle , then stop.
Then you have the upper border.
If you begin low, you might end up with to low pressure for speed.

 

HOI ShIzNiT!



Bro, you are *WAY* over thinking and that math would be ballpark at best, way too many variables.

For reference I am over 6k lbs and if my tires (285 75 17 LT E load ST Maxx) are over 35psi it rides like a dump truck on a road paved with only speed bumps.

Do the chalk test, and small incremental increases (like 1psi) do have noticeable differences in handling.

I usually run 31psi per my chalk test, front and rear, and 33psi if I am long hauling, improves MPG slightly without making ride too harsh but also decreases wet traction so round town I stay at 31psi for comfort and traction.

I can't imagine rolling on pneumatic rocks at 40PSI.

Let alone 70, and 80 is asking for trouble, seriously you could have a catastrophic tire deflation. (Tire explodes, this kills people bro.)

For what it's worth I believe the maximum pressure on my tires is 60psi and that's for like massive loaded down dual diesel tow pigs pushing 10k lbs.

Just do the chalk test fam.

 

Well thank god I didnt blow myself up driving across country without finding the tire pressure formula I was looking for. 1600 miles at 44 psi. Ended up weighing in close to 6900lbs. Rode great at 44. Will air down now that I am past the majority of interstate travel.
Chalk test showed a hair of chalk left on the outer edges at this psi.

 

Damn, pushing 7000, you beefed up your frame at all?

What all you got in that to make it so heavy?

I'm similarly equiped to the picture of yours externally and I'm barely at 6k fully loaded down.

Can only imagine that thing is a dog on the highway. What kind of gallons per mile do you get?