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Thread: Tire Pressure

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    Tire Pressure

    I am under the impression that people make a very common mistake when it comes to how much air to put in their vehicle tires. Back in the day, I would simply look at what’s written on the tire regarding PSI (pounds per square inch). I might put a few pounds less in if my driving conditions warranted it. It was generally a good rule of thumb...but perhaps only coincidentally so.

    The truth of the matter (as I’ve come to grasp and recollect again) is that the answer for the right amount of air pressure is found somewhere but NOT on the tire. All the tires tell us is the MAX PSI for that particular tire, not the correct or ideal amount of air for the tires that so happen to be on the vehicle. In others word, if you want to know about the max psi that is recommended for my specific tires, then look specifically at the tires. If you want to know the best tire pressure for the vehicle, then look at the sticker on the drivers door or owners manual.

    That being said, one of my vehicles is an F-350 (that’s not a duly) and the tires say 65 max psi. The door says 65psi for front and 75psi for back.

    Based on that, I should have (if my thinking is straight) 65psi in all four tires.

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    Fair dinkum thinkum bilby's Avatar
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    Quote Originally Posted by fast View Post
    I am under the impression that people make a very common mistake when it comes to how much air to put in their vehicle tires. Back in the day, I would simply look at what’s written on the tire regarding PSI (pounds per square inch). I might put a few pounds less in if my driving conditions warranted it. It was generally a good rule of thumb...but perhaps only coincidentally so.

    The truth of the matter (as I’ve come to grasp and recollect again) is that the answer for the right amount of air pressure is found somewhere but NOT on the tire. All the tires tell us is the MAX PSI for that particular tire, not the correct or ideal amount of air for the tires that so happen to be on the vehicle. In others word, if you want to know about the max psi that is recommended for my specific tires, then look specifically at the tires. If you want to know the best tire pressure for the vehicle, then look at the sticker on the drivers door or owners manual.

    That being said, one of my vehicles is an F-350 (that’s not a duly) and the tires say 65 max psi. The door says 65psi for front and 75psi for back.

    Based on that, I should have (if my thinking is straight) 65psi in all four tires.
    Based on that, you should get tyres that are appropriately rated for the vehicle.

    It sounds to me like you have an underrated set of tyres, intended for use on lighter vehicles. Buy tyres with a max pressure rating of 80psi or more, and inflate them to 65 on the front and 75 on the back.

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    Veteran Member Wiploc's Avatar
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    My belief is this: If you have the axel loaded to maximum legal capacity, then you should have the tires inflated to the legal maximum (per the tire and the door). But if you have only half that much weight on the tire, you should have only half that much pressure in the tire.

    Not that I check the axle weight or do that math, but that's what I was once told I should do.

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    Super Moderator Bronzeage's Avatar
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    The pressure stamped on the tire is the maximum. The pressures listed on the vehicle label are a compromise between handling quality, ride quality, and most importantly, tire longevity.

    If anyone remembers the scandal about Ford Explorers and Firestone Wilderness tires (There was an above average number of accidents with injuries caused by sudden tire failures), the root cause was found to be owners who had driven 15,000 to 20,000 miles with under inflated tires. This caused the sidewalls to weaken before the tread wore out. This is also the reason all new cars have tire pressure monitoring systems. Tire pressure is a very simple act of automobile maintenance, but in the 21st century, it's too much to ask.

    When I worked in a Toyota service department, Toyota's T-100 pickup specified a lower pressure (24PSI, if I remember correctly) on the front than on the back. This meant tire pressure had to be changed a good deal after a tire rotation. This was a constant problem. A Toyota owner would come in complaining about rough ride (hot coffee in the cup holder would splash out) and it was always after getting an oil change and tire rotation.

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    Quote Originally Posted by Wiploc View Post
    My belief is this: If you have the axel loaded to maximum legal capacity, then you should have the tires inflated to the legal maximum (per the tire and the door). But if you have only half that much weight on the tire, you should have only half that much pressure in the tire.

    Not that I check the axle weight or do that math, but that's what I was once told I should do.
    I had it in the back of my mind that tire pressure was also a function of weight. Here’s the thing, I have a 40 foot slide out that right now is infrequently used, so it’s optimal to lean towards having tire pressure be a function of unloaded weight—not the fully loaded weight as indicated by the sticker. These particular tires have some very high ply rating too, so they can withstand missing the mark, but I haven’t the foggiest idea how to calculate it ... yet.

    I’m thinking that I actually should get my numbers together and find a formula that incorporates truck weight, coolant, gas, oil, driver, passenger, cargo, curb weight, scaled weight, and total weight. You’d think if it were a big deal, the sticker would give a range from minimal cargo to maximum cargo.

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    Veteran Member TV and credit cards's Avatar
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    fast, you have "Load Range D" (max. 65 psi cold) tires on a truck that was made for "Load Range E" (max. 80 psi cold) tires. This is the maximum pressure the tire is designed to handle. It should not be referred to for proper inflation. Always the doorjamb info. The truck was made for heavy loads and the tires correspondingly.


    Basic Tire Info


    Always go by what's on the sticker in the driver's doorjamb and buy tires the vehicle manufacturer recommends. Any tire retailer's website will steer you in the right direction. When you go outside the manufacturer's recommendation, you might be having to guess at proper pressure. Avoid the knowledge base of your local tire installer. He's making twelve dollars an hour for a reason. Consult your owner's manual about heavy loads. If you should increase pressure for towing or maxing out the load, it will tell you. I doubt you should but I've rarely had to concern myself with such. Always go with the owner's manual. I wouldn't second guess the people who made the vehicle. They already did the math.

    Under inflated tires will have more rolling resistance and obviously will heat up more while driving.

    Over inflated tires may not be making full contact with the road while driving, riding high in the center. Not so good either when traction is a concern and it always is.

    To whom it may concern, if you're unfamiliar with winter tires like Bridgestone Blizzak, they are wonderful. I buy from the local Walmart and have them swap out my tires every Nov/Apr 15th. I'm actually impressed with Walmart in these regards. They properly torque the lugs, a second employee comes by and checks the torque, the vehicle is then taken out and driven in a figure eight behind the store, brought in and checked again. I of course check the torque after about 100 miles driving as we all should. Sadly, there's not a Lug Torque Monitoring System.
    Dwight

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    Veteran Member Wiploc's Avatar
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    We bought a fifth wheel and a pickup truck to tow it. Our first outing was to a Life on Wheels convention.

    People there weighed us at each wheel, and explained that we would never be street legal unless we traded up to a medium duty truck, which we didn't want to do.

    We wound up getting a Jayco Seneca motor home. Sold our house, got rid of everything that wouldn't go in the motor home (except for a little bit that when into storage in relative's basements) put all our remaining belongings in the motor home, filled it with water and fuel and groceries and ourselves, and then went to the scale.

    We were 1100 pounds under the cargo carrying capacity. That is to say, we were street legal by a wide margin.

    We got in the habit, at RV shows, of always asking where the CCC (cargo carrying capacity) was immediately upon entering. The salesman would be embarrassed not to know, but no other factor would be more important to the sale. The ones like Jayco, that had street legal units, should have been trumpeting that. I saw one with a 70 pound CCC. My wife saw one Bigfoot with a 40 pound CCC.

    (These CCCs are calculated assuming that I weigh 150 pounds, which is not the case.)

    We saw a big class A, like a Greyhound bus, with a CCC of just 400 pounds. It had all that gorgeous belly space, and no legal way to use it.

    Anyway, here's a picture, I hope:

    You want your tire shaped like that third image. You get that shape if you are fully loaded and fully inflated. You also get that shape if you are 80% loaded and 80% inflated, and so on like that.

    I'm not saying this on my own authority. I'm trying to parrot something I heard a long time ago. But I think I've got it right.

    I wouldn't worry about having the slide out; you're not going to run with the slide out. You want that proper tire shape while running down the highway, not while parked.

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    It’s coming together. I wonder if ply count alters the poundage necessary for proper inflation.

    ETA: the cargo poundage

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    Fair dinkum thinkum bilby's Avatar
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    Quote Originally Posted by fast View Post
    It’s coming together. I wonder if ply count alters the poundage necessary for proper inflation.

    ETA: the cargo poundage
    You don't need to wonder about such things; The engineers at Ford did all that on your behalf decades ago, and boiled down the answers to some simple information most of which is in the manual, and the rest of which you can get from your local Ford dealer.

    Fit tyres that match the ratings recommended by the vehicle manufacturer, and inflate them to the pressure recommended by them, and displayed on the placard attached to the vehicle.

    There's no need for you to reinvent the wheel (or the tyre).

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    Quote Originally Posted by bilby View Post
    Quote Originally Posted by fast View Post
    It’s coming together. I wonder if ply count alters the poundage necessary for proper inflation.

    ETA: the cargo poundage
    You don't need to wonder about such things; The engineers at Ford did all that on your behalf decades ago, and boiled down the answers to some simple information most of which is in the manual, and the rest of which you can get from your local Ford dealer.

    Fit tyres that match the ratings recommended by the vehicle manufacturer, and inflate them to the pressure recommended by them, and displayed on the placard attached to the vehicle.

    There's no need for you to reinvent the wheel (or the tyre).
    And there's no need for you guys to reinvent the spelling of tire!

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