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