How do you determine a correct tire pressure?

...and if your tire is not mounted on a measured rim width wheel for that particular tire size, section width will change; people's contact patch expectations do not.
Change rim width by 1", section width expectation should change by 1/2".
I can easily see people over- or under-inflating trying to dial in that last 1/2" of contact patch.

 

Yeah, if you're at the extremes of the ranges it'll let you know.

That said if I'm keeping the same tire size and load rating and am just loading up for a trip, I can get a good estimate of how much air to put in, some very quick math without even getting out of my chair will help me.

If you've got a 4500 lb truck and the OEM psi is 32 psi, 4500 lb / 32 / 4 = ~35 sq inches of contact patch on each tire.

Now add 1100 lbs of junk to the truck and maintain the same contact patch size...

5600 lbs / 4 / 35 = 40 psi

So that would be an extra 8 psi in each tire, in a scenario where we're at GVWR.

 

I've kept my tires at 36 psi since I started driving in 1986. FWD, AWD, 4WD, RWD, sedans, hatchbacks, pickups, SUVs and one convertible.

That's where my tires were set when I got my Tacoma back in Feb.

Don't see a need to change.

 

You are correct...
The TREAD Act ruling states the recommended psi is for maximum loaded condition.

 

Safety and optimum tire wear are reason enough for me.

 

I notice you have a 2000?
Things changed mid-2000s regarding door placard psi...
I'm just curious about the info, since it pre-dates the changes and I'm not that familiar with 1st gens. Could you post a pic?

 

Yep...definitely pre-TREAD.
Pre-TREAD was based more on 'normal' loading conditions and had a lower dynamic load factor of 1.33, based off GVWR; compared to 2nd and 3rd gen, which have a dynamic load factor of 1.54.

You are wise to inflate for significant weight increases.

 

Interesting, first I heard of it.

Was TREAD in response to the whole ford explorer fiasco?

 

Yes.
https://www.federalregister.gov/documents/2002/11/18/02-28682/tire-safety-information

 

From pressure rising you can calculate the temperature in the tire.
So measuring warm can be usefull.

When driving a constant speed of 50mph, at an ambiënt temperature of 65 degr F, temp in tire rises to about 110 to 115 degr F, wich gives about 11% rise in pressure.

But external factors , like sunshinebon tire, extreme ambiënt temp, to much friction in tire, severe braking, can rise the temp in tire extra. As long as the rubber dont reach its critical temperature, no problem.

 

Yikes, try 35psi, see how it rides, measure fuel milage, if not, try 32psi, see how it rides, measure fuel milage. You'll find what the tires like, what the truck like, and what you like for ride compared to tire pressure.

 

Okay, it's been a few days; work, rainy weather, home, wife, etc. You get it.

So, I finally got around to doing the chalk test. But, I'm keeping in mind the link to the online calculator that generated a 35psi based on previous tire for the vehicle. I can't begin to argue bias vs radial tires for test validity. However, for me I'm looking for a good contact patch that indicates the right sort of wear and grip for the tire, roughly speaking. To be sure, we are roughly speaking aren't we? Pressures will vary anyway due to a number of factors so, I'm reasonably certain that I'm in a good range for mileage, safety, and wear. Again, wear was my biggest concern...they're expensive!

I started with a good chalk patch. For those that saw this from the beginning, I was able to man-up enough to beat up the neighbor kid and take her chalk. I'm swelling with pride!


Pressure (cold) was at 44psi. To be fair, I told the Toytec fellas to "max" tire pressure until I figured it out. Below is the first bit of wear at 44psi:


tried it again at 40psi. Close but a little more...



Here we are at 36psi. Bias or radial arguments aside, for me it is interesting that the calculated pressure and the chalk test appear to coincide. I could likely go up or down a pound or two with little consequence. This is exactly what I wanted. Thanks all for the input.

 

Looks good, try 34 psi, see where that takes you, might ride better....

 

I can see where that might work for some. Personally, I can't even tell the difference between 44psi and 36psi, let alone 2psi. I guess my butt isn't as sensitive to ride quality as some!

 

275-70-17 8.5 wheels
I run 32 front 30 back with no load in the bed