Tire pressure formula

[randomhero439]

I like my tire pressures a bit higher than the stock pressure recommendations. The first of every month I set my tire pressures. Ive been using this formula on my cars, and thought I would share it with BITOG. This works best at night when there is no sun and the temperature has stabilized. Never go over the PSI posted on the side of your tire wall. The monthly record low temps can be found on the Weather Channel website.

(Current temperature - Record low temperature) / 10 + Specified tire pressure

Here is an example of my tire pressure for January:
(17 - (-20)) / 10 + 32 = 35.7psi rounded up to 36psi

For people who set their pressures very high to increase fuel economy and other things:
(Current temperature - Record high temperature) / 10 + Max tire pressure(the number on the side wall)

 

[Johnny248]

Why not just set the psi to the vehicle placard with cold tires in the morning or at night?

 

[GSCJR]

^^^^

 

[Shannow]

PV=nRT, but you have to go back to absolute pressure and temperature

for a tyre, P1/T1 = P2/T2

 

[Donald]

Originally Posted By: Johnny248
Why not just set the psi to the vehicle placard with cold tires in the morning or at night?


+1

or sometimes if I am filling hot tires at a gas station and close to home, I fill them a few pounds over and do the final adjustment at home the next morning.

 

[randomhero439]

My method compensates for extreme cold weather.

For example if you set your tires at 32psi when its 20 degrees out, they will be around 34psi during the day if the temp rises 20 degrees.

However, what if you get a massive cold front like we just had in Ohio where temps were in the single digits, then your tires are "low."

Personally, i prefer my tires at higher pressures. The factory place card is a reference that factors in NVH, comfort, grip, responsiveness, and longevity (usually in that order). By increasing the pressure a bit, it reverses that order. But i prefer it like that

 

[toneydoc]

My formula is simply to look at my Slime digital gauge and when it says 35.0 in the tire, quit adding air and move on to the next one. Works like a charm.

 

[SwampSurvivor]

If the tire is wearing in the center, I lower the pressure. If it's wearing on the outside, I give it some air. Pretty simple, I think.

If I'm going to be carrying a big load or passengers, it's set to the max on the sidewall.

 

[RiceCake]

I can all this math and just inflate them to the maximum PSI listed on the tire sidewall. That's what the tires are rated for, cold, so I inflate them to that, cold.

44 PSI in all 4 versus the 30 PSI the car calls for. Tires last longer, MPG increases. Course, handling becomes more loose and ride harshness increases exponentially; but in my car I'm fine with it.

 

[GSCJR]

Just buy a Nissan...they let you know when to stop.

I just had to do this...LOL

 

[Garak]

Originally Posted By: GSCJR
Just buy a Nissan...they let you know when to stop.


Not my G.

The TPMS is so sensitive that I have enough issues that I certainly understand why the OP is doing what he's doing. There is a major difference between the heated garage at 9 C versus outside at -30.

 

[randomhero439]

Originally Posted By: Garak
Originally Posted By: GSCJR
Just buy a Nissan...they let you know when to stop.


Not my G.

The TPMS is so sensitive that I have enough issues that I certainly understand why the OP is doing what he's doing. There is a major difference between the heated garage at 9 C versus outside at -30.


Thanks, i would not want to set tire pressures at -30C lol
So in this case. (49-(-22)/10+35? equals 42psi in the garage, and the tires drop to 35psi outside. This formula just compensates for the chance of the coldest possible temperature.

 

[Eddie]

I try not to overthink stuff anymore. I set my tires overnight cold and add 2 psig. This once a month or before any long highway trip at 65+ mph. Ed

 

[Garak]

I didn't bother trying to calculate, but I do add a few pounds when it's that cold outside. If I set it at 33 in the garage and it's cold out, the TPMS grumbles in a rather short while. And I'm not leaving it outside, then pulling into the garage just to check pressure, nor am I going to play around at a gas station in -30 when I have a perfectly good air compressor in a heated garage.

The TPMS is sensitive, so I do have to compensate.

 

[mechtech2]

I wish they did not print pressures on sidewalls.
Nobody can read the few surrounding words.
Like, @ 'X' weight.

 

[Shannow]

Originally Posted By: RiceCake
I can all this math and just inflate them to the maximum PSI listed on the tire sidewall. That's what the tires are rated for, cold, so I inflate them to that, cold.

44 PSI in all 4 versus the 30 PSI the car calls for. Tires last longer, MPG increases. Course, handling becomes more loose and ride harshness increases exponentially; but in my car I'm fine with it.


But the engineers who built your car recommended an inflation pressure, knowing full well that the tyres are rated to a higher pressure (safety factor)...how can you logically argue against their recommendation, when they know best, and were there to be any gain in mileage (versus your acknowledged reduction in safety), they would be onto it like white on rice ?

 

[SteveSRT8]

^^^Exactly.

Ricecake is endangering himself, his passengers, and other drivers on the road. Way too much risk for me.

If anyone is ever involved in a serious accident there may be scrutiny of the various parts of the car. I have seen an inspector look at brakes, steering gear, etc. when big money was on the table (lawsuit). Who's to say they couldn't take tire pressure?

 

[CapriRacer]

Let me reiterate what's been said about the maximum pressure listed on the sidewall of a tire:

DO NOT USE THE PRESSURE LISTED ON THE SIDEWALL!

(unless that's what the vehicle tire placard says to use)

The pressure is a maximum. It is not a recommendation.

Also, the maximum pressure is not always tied to the max load. Sometimes the 2 are independent of each other.

 

[RiceCake]

Originally Posted By: Shannow
Originally Posted By: RiceCake
I can all this math and just inflate them to the maximum PSI listed on the tire sidewall. That's what the tires are rated for, cold, so I inflate them to that, cold.

44 PSI in all 4 versus the 30 PSI the car calls for. Tires last longer, MPG increases. Course, handling becomes more loose and ride harshness increases exponentially; but in my car I'm fine with it.


But the engineers who built your car recommended an inflation pressure, knowing full well that the tyres are rated to a higher pressure (safety factor)...how can you logically argue against their recommendation, when they know best, and were there to be any gain in mileage (versus your acknowledged reduction in safety), they would be onto it like white on rice ?


Not really. The tire pressure is primarily designed around a multiplex of variables, mainly keeping the tires firm enough to support the vehicle's entire curb weight, but soft enough to maintain handling performance. Increasing the pressure means the tire can handle more weight, and the possibility of increasing centre treadwear if you're not carrying that weight, but I haven't had that happen yet. Contrary to popular sensationalist belief around here tire pressure isn't unanimously designed with fuel economy in mind. It is with LRR tires on economical vehicles, though, which often run upwards of 50 PSI in their tires, but your everyday cars typically are calibrated to run lower pressure to improve ride harshness and noise. Of course, you can inflate beyond that, but you start dipping into other variables. IE: noise, handling performance, etcetra.

Originally Posted By: mechtech2
I wish they did not print pressures on sidewalls.
Nobody can read the few surrounding words.
Like, @ 'X' weight.


Mine say nothing about weight. Besides, there's nothing wrong with running a tire overpressure (but not beyond the maximum specification). Its not optimal though by any means for most situations. But just because a tire at 40 PSI can hold 1,600 Lbs or something and the stock weight is 1,400 on 35 isn't a big deal by any means. Its not like your tire is suddenly going to erupt because you're not weighing it down accordingly.

 

[RiceCake]

Originally Posted By: CapriRacer
Let me reiterate what's been said about the maximum pressure listed on the sidewall of a tire:

DO NOT USE THE PRESSURE LISTED ON THE SIDEWALL!

(unless that's what the vehicle tire placard says to use)


^ I concur with this by the way...for most people. Its a bad yardstick for inflation without judging a number of other factors into your decision, most notably the often dramatic levels in reduction of grip.