Home > auto > What should my tire pressure be, if I changed my original tires?

What should my tire pressure be, if I changed my original tires?

September 1, 2011 Leave a comment Go to comments

Please keep in mind all this is purely my opinion, read it as entertainment value and please do you own research before committing to some random guy’s opinion on something that can potentially affect your life and well-being.  I decided to write today’s blog on this specific question because I got into a heated argument with a mechanic while I was getting my oil changed, and I figure this is a safety concern that EVERYBODY needs to know about.  The mechanic was telling me I was in danger because my tires were way over inflated.  Little did he know who he was dealing with.  I responded immediately that HE could have potentially get me in to an accident because he deflated my tires.  You have higher probably of tire failure with under inflated tire vs. over inflated tire.  Under inflated tires generate lot more heat, because of higher friction and resistance between the rubber and the road, and cause your rubber to break down both inside and outside, it’s a very dangerous situation. 

The mechanic said the tire pressure needs to be set based on what the vehicle’s requirement is set to.  This is typically written on a sticker by the driver’s door.  I told him that is absolutely true if the car has the recommended factory set of tires, or equivalent.  I specifically told the mechanic that I had upgraded my tires to a larger aftermarket tires and it has different set of requirements from the original car factory spec.  He was insisting that was irrelevant and it needs to adhere to the original car factory spec.  Long story short, I told him to leave my tires alone and I ended getting the oil change I needed.  So who is right?

The factory set pressure number only applies to factory set equipment.  Its what they designed to, its what they analyzed to, its what they tested to, and its what correlates back to all their calculations.  Factory spec applies to factory set/recommended equipment!! if you change anything in your car outside of the factory spec, it may or may not apply.  

So what do you do if you go against the factory recommendation and upgrade to some really big heavy duty tires with completely different PSI range?  Do you just randomly set it to a number based on the tire range or auto factory range or something in between? The answer is a big “NO”.  You, or qualified, tech/engineer must determine that number.  Since most drivers in this country will not have state of the art testing equipment, sensors, and DAQ we will have to improvise. 

Your ultimate goal is to set the pressure such that there is even pressure across the width of the rubber that is in contact with the road.  There are many different ways to figure this out using just the tools you have at home.  Just be sure your tires are at ambient temperature before continuing with the exercise.  Here are some of my suggestions to determine the pressure gradient:

1.  Use sharpie/markers/chalk to draw some lines across your tires and drive it for a while.  You will start to see some areas start to rub of more than other areas.

2.  Use a very thin plastic sheet like the overhead transparency sheet for projectors.  Use pair of scissors and cut them in strips and lay them down on a flat surface and roll your car over it.  Make sure its sandwiched between your tire and a flat surface.  In an extreme case you may be able to pull some of these strips out, but most likely you wont be able to pull any of these out.  Let your car rest on it for few minutes and roll your car off it.  You should be able to see all the indentations on the strips and determine which area has the high/low pressure.

Now that you know what the high/low spots are, you can inflate/deflate using this information.  If you find there is more pressure in the center region of the tire, it means you have too much air in the tire and its starting to balloon up.  Release some pressure and run the test again.  If you find you have more pressure toward the ends of the tire, you have too little pressure, this is very dangerous!! Add more air in to remedy this.  You definitely want to have more air than less air, but you want to try to shoot for nominal, which is nice flat contact across the rubber.  Also, DO NOT EXCEED the max limit of the tire, you should be well below it.  Check your pressure and check it often, especially with changing weather condition.

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Categories: auto
  1. Phil
    September 10, 2011 at 11:35 pm

    Never let your mechanic (or anyone else) touch anything except what you tell them to. Here’s a potentially sad tale I caught in time. I got a set of load range C Michelins for my work van at Tire Kingdom, making sure they marked the service order for 50 PSI (the Michelin’s max pressure) because of the maximum load I carry. I pull out and make a right turn and the van slews all over- I couldn’t keep it in my lane at 10 MPH!. I immediately pull off into another lot and grab my gauge. 35 PSI in all tires. I go back and they ‘fix the problem’ apologetically. One worn out set of Michelins replaced later, same van, same store, SAME THING. It doesn’t take a lot of brains to operate a tire machine or balancer and that is the level of intelligence found at any tire store- anyone smart enough to have a better job is working elsewhere!

    I have 30+ years experience driving pick-ups, cars, trucks, vans, and motorcycles under every condition except track racing (though I’ve pit crewed many times). Here’s the real scoop about proper tire pressure- From cold, a properly inflated tire will see about a 10% rise in pressure under use (hot pressure checked immediately after stopping) when the pressure is correct. This rule works on the road, off the road, loaded, unloaded, motocross, motorcycle, high speed and low speed. car, truck, whatever. From this point you may find better results from a 5% lower pressure to 10% higher pressure; this being based on need (ie a better ride or better cornering/braking, better economy etc.) It works for everything and is easy to do- just check cold, do whatever you normally do, check hot and see how much the pressure rose. Too high a rise = underinflation, to low a rise indicates overinflation.

    With a quality tire like the major brands (and why are you trusting your life on anything less?) you can safely exceed the cold max by 10%. I’ve ran 25% over a number of times with no ill effect other than handling just as an experiment (except motorcycles where I never exceed the Max pressure). The tire pressure info from the vehicle manufacturer means nothing even with stock equipment ( remember the 22-24 PSI rear pressure in the Explorer? Ford did that only to get a better ride quality- though personally I don’t think rolling over at highway speeds is comfortable at all! Firestone designerd this tire with a 27 PSI minimum, bending it to Ford’s wishes to approve the underinflation so they’d get the contract) What they recommend applies only to their idea of an average buyer and user, meaning someone driving at the speed limit on dry roads with a total of 1 1/2 people in the car and half a tank of fuel (an often un-thought of variable in this equation). Do anything else and all bets (and recommended pressures) are off!

    My van has 52 cold in the rear and 46 cold in front with a 50psi max cold rating, checked weekly or more often. This gives me the best stability, load capacity, handling and wear. Am I worried? Heck no. A quality tire isn’t going to fail over a 2 psi overpressure. It’s not going to fail over a 5 psi over rating. If a pressure related failure happens it will be because of underpressure or grossly exceeding max pressure. I have so much traction that even on wet roads I’d roll over before I lost cornering traction because of my heavy and high-centered load. Were I to dump it all and drive empty I’d likely see a 15 PSI change needed for optimum everything.With 40 gallons of fuel on board I can feel the difference in handling versus when running on fumes. The biggest offenders in tire pressure mistakes is pick-up trucks, which go from empty to over-loaded with no thought given to tire pressure. When I see a loaded pick-up on the road I always look at the rear tires when I’m near them because of this danger. Most of them are obviously underinflated so I give them a wide berth- that trick has saved my hide several times as their tire blew or control was lost. If you drive a pick-up you need a compressor with you to adjust for load, but nobody sees to understand that.

    If you aren’t sure what the pressure should be it’s usually best to go half-way between the manufacturer’s recommended pressure and the max cold rating if you can’t do the 10% test, going to max cold for a heavy load. Drive cautiously and all will be well. Those four (or on a bike two) little patches of rubber touching the asphalt are the only things between you and death, so view the situation accordingly knowing that underinflation is far more deadly than overinflation.

    • September 13, 2011 at 7:39 pm

      Well said, sir. Folks, please take your mechanic’s advice with a grain of salt.

  2. February 6, 2012 at 9:29 am

    thank’s for this blog.

    • February 6, 2012 at 1:11 pm

      Thanks for the complement. Tire pressure is a pretty serious thing to monitor.

  3. September 28, 2015 at 4:04 am

    Hello,

    Cool article. I usually run my tires a bit above the “car recommendation (2.5bar or 36psi).” I just changed tires and went from 205/40/18 to 215/40/18. How much should i increase the tire pressure above the “car recommendation?”

  1. September 5, 2015 at 7:28 pm

speak up, can't hear you!

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