How to calculate street pressure for larger tires.

[javajoe79]

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.

Your last sentence contradicts the rest of your post it seems... I don't think it's necessarily so cut and dry. Like I posted before, each tire has a different spring rate at a given pressure. On the road at speed, we want the correct pressure so as to not over heat the tire and to provide the proper contact patch. That pressure will change from tire to tire.

 

[eggman918]

Different rim widths on the same tire make a difference as well

 

[jk_surgeonfish]

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!

What is considered too hot? Wouldn't checking in winter show different values then in heat of summer and therefore how would you know what is running too hot?

Thanks!

 

[javajoe79]

You could get away with lower pressures in winter with the added cooling effect but what you would look for is an increasing temperature over time. If you touch a tire after running at high speed for a while and it's too hot to touch, it's probably running too hot.

 

[downhill]

What is considered too hot? Wouldn't checking in winter show different values then in heat of summer and therefore how would you know what is running too hot?

Thanks!

The amount of heat that a tire can take will vary depending on construction, but some TPMS sensor systems have a heat warning at 156 degrees. I guess that is probably "too hot". Obviously, inflation and ambient temp figures into that. That is why most blowouts happen on hot summer days. The temperature of the tire, will be the sum of the ambient temperature and the temperature generated by internal friction from tire flexing. The whole TPMS thing got started as a result of Firestone tires being run at low inflation on Ford vehicles. Firestone took the biggest hit, but Ford was likely more to blame for suggesting the lower pressures.

 

[steelhd]

Chalk test. Really simple and 100% effective.

 

[Happy Joe]

What is considered too hot? Wouldn't checking in winter show different values then in heat of summer and therefore how would you know what is running too hot?

Thanks!

When the tire is uncomfortably hot to my touch I check for (and find a leak) which has caused low pressure/high temperature; Note; you have to check immediately after stopping from a highway speed run or the tire will cool.
Yes; this is not effective in winter... Its just a quick and dirty (hand) general check.
Check them now after a highway run you will get experience and , hopefully, just find them warm.
Enjoy!

 

[NatersXJ6]

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.

I'm pretty sure that when they do those space shots, they just have Bruce Willis and Steve Buscemi go rub chalk on the asteroid until it rubs off evenly... I seem to remember seeing that in a documentary film once.

I too, simply chalk the tires.

 

[Meili]

Here is what I do to get a BASELINE for the chalk test.

You need to know your front and rear axle weights. No one is running the same pressure front and rear, right?

Example:

5000 lb vehicle with 60/40 weight distribution
3000 lb front axle or 1500 lb per tire
2000 lb rear axle or 1000 lb per tire

Each tire is rated at 3050 lb max load @ 80 PSI
3050/80= 38 so each 1 lb of air supports 38 lbs (real world is not linear but this works here)

So each front tire supports 1500 lbs, 1500/38 = 39.5 psi
Since each rear tire supports 1000 lbs, 1000/38 = 26 psi

This gets you a BASELINE for the chalk test. Test and adjust accordingly.

I have never run less then 4 "extra" lbs in the front on any vehicle I have owned.

https://www.google.com/url?sa=t&rct...58MQt4&usg=AFQjCNGRwja02oW0vd6qI4xjh5KZTiMGWw

 

[steelhd]

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Did you intend to link a never ending advertisement for Armageddon?

 

[Scoutn79]

My math tells me moving from a H78-15 to a 33x12.5x15 (41.3% wider and 35.7% more sidewall = 77% difference.
It recommends 7.36 psi. I don't even run that low on the trail....
The chalk test tells me 30 psi (truck is a bit heavier now) and the tires wear almost perfectly evenly across the tread.

Darrell

 

[Happy Joe]

I would never figure tire pressure that way; it takes no account of the tire loading or vehicle.
I take the weight of the vehicle, including the average load, per tire then factor it against the load rating of the new tires at max/sidewall psi to get a ball park. (usually in the 20 to 27 psi range) then run a chalk test to get the what the tires like for even tread contact; to get the final pressure (keeping historical safe pressures in mind).
Then do a 20 mile or so interstate/highway run and verify that the tires are not overheating.
Meili (above) has the right idea...

Enjoy!

 

[REasley]

I really hesitated to comment on this thread, but I do it a little different. When I put on a new set of tires, I experiment with different pressures to find the the most comfortable and stable ride. Then I watch the tread wear carefully and adjust accordingly. I also monitor the tire temperature and any time the temperature rises to or above 135* I either raise the pressure or slow down. When pulling my 5th wheel it's always "go slower" as I am running at or near max. I am more concern with the ride and handling, not so concerned about getting maximum tread wear. I would prefer to buy new tires than suffer with an overly stiff ride.

 

[Stumpalump]

I really hesitated to comment on this thread, but I do it a little different. When I put on a new set of tires, I experiment with different pressures to find the the most comfortable and stable ride. Then I watch the tread wear carefully and adjust accordingly. I also monitor the tire temperature and any time the temperature rises to or above 135* I either raise the pressure or slow down. When pulling my 5th wheel it's always "go slower" as I am running at or near max. I am more concern with the ride and handling, not so concerned about getting maximum tread wear. I would prefer to buy new tires than suffer with an overly stiff ride.

Too much air beats up the suspension components. Too soft is bad in case of evasive maneuver....I do check temps at gas stops. Last month returning from Death Valley at night one felt warm in Wikieup. It was seperating. Michelin tire on the van. I limped home on it because the spare was an odd size and 10 years old. New tire and new spare. Who needs the agrivation of a crappy spare when your loaded down. Next trip to Glamis and one trailer tire felt warmer in Quartzite AZ. 20 PSI was in it along with a nail. Plug kit and compressor made a quick permanent fix. You can really feel a heat difference if something is not right. While stoped for a break a quick hand on the tires saved me twice in one month.

 

[e60ral]

Here is the correct way to do it:

If the customer does not have or is not putting the OE tire size on the vehicle, determine air pressure by doing the following:
1. Use vehicle placard or Tire Guide to find load index and air pressure of OE tire.
2. Select the correct inflation table and find the load carrying capacity of the OE tire.
3. On the replacement tire you recommended, find the load index (use the store system or the tire itself).
4. Use the inflation table that corresponds to the replacement tire's load range. On the selected inflation table:

X Find the replacement tire's load index line.
X Move across that row until you find the load that is equal to or just greater than the OE load recommendation.
X The recommended air pressure is at the top of that column.​

https://static1.1.sqspcdn.com/static/f/589830/23447320/1378330097907/Discount+Tire+inflation.pdf

if you 4 corner weigh your vehicle fully loaded you can fine tune this based on your load requirements, but guesstimating is not enough. the "divide load by psi" method is wrong (its close, but incorrect because the load-inflation intercept is not at 0). chalking is good to show wear, and will usually correlate with proper inflation, but is not enough on its own without verifying the load.