tire pressure: actual vs recommended
- Thread starter wbwanzer
- Start date
No I was careful to make the comparison for the same weight and tyre size. The only possible explanation for recommending higher pressures is to reduce rolling resistance which is a pretty fruitless exercise.
Auto Express car magazine did a real test by dropping tyre pressures from the recommended 39 psi to 20 psi which being a near 50% reduction in pressure you would expect it to make a huge difference. What happened over a carefully replicated test was mpg dropped from 30.5 mpg to 29.7 mpg. They were so surprised they repeated the test and got the same result. Now I'm not advocating such huge reductions because that would be dangerous but it demonstrates that any savings from going above placard pressure must be infinitesimal.
Door says 35 psi....I always go 35.2 psi.... 
Nowadays often recomended pressures are kept verry high , for liability- and energysaving-reasons.
The referencepressure ,is pressure for wich, together with reference-speed of 160 kmph/ 99 mph, the maxload is calculated and given on sidewall, is for a standard load personscartire 36 psi in eur system and 35 psi in US system , in case of daubt always use the higher.
XL/reinforced/extraload EUR 42 psi , US 41 psi.
So if you see one of those recomended, its most likely not calculated for max technical carspeed and GAWR, wich they mostly only give nowadays.
Then if calculated, it might come to much lower , because tires have a comfortable reserve in maxload mayby in the 20s psi, and real axleloads are lower then GAWR's in normal use, wich even yustifies a lower pressure.
Rollingresistance makes small part of total energy use, so influence of tirepressure on fuel/energy-consumption is not that much.
You can reach more energy saving with driving carefully .
So adding 4 psi to the already high recomended pressure is not needed.
If you calculate it yourselfes with use of 99% acurate determined axleloads in your use ( succes with that, the most tricky part, and your responcibility) you can add 10% for reserve , and if you use maxload in the lineair calc, it still gives acceptable comfort and gripp, and max speed, wich you wont go over for even a minute 99 mph, not that hard to stay below.
Even if calculated for GAWR's it might come to in the 20s psi.
Formula for lineair calc, upto 99mph( official calc comes to slighty lower pressure) is next.
(( real axleload+10%)/( 2x maxload of tire) x reference-pressure= to use cold pressure.
Then fill that cold at about 20 degrC/ 68 degrF, and let it flow cold and warm, with ambiënt temperature.
Main goal of tiremakers,is to not overheat any part of tire-material, when driving the speed constantly, for wich deflection of tire is determined, so pressure is calculated for the load.
The referencepressure ,is pressure for wich, together with reference-speed of 160 kmph/ 99 mph, the maxload is calculated and given on sidewall, is for a standard load personscartire 36 psi in eur system and 35 psi in US system , in case of daubt always use the higher.
XL/reinforced/extraload EUR 42 psi , US 41 psi.
So if you see one of those recomended, its most likely not calculated for max technical carspeed and GAWR, wich they mostly only give nowadays.
Then if calculated, it might come to much lower , because tires have a comfortable reserve in maxload mayby in the 20s psi, and real axleloads are lower then GAWR's in normal use, wich even yustifies a lower pressure.
Rollingresistance makes small part of total energy use, so influence of tirepressure on fuel/energy-consumption is not that much.
You can reach more energy saving with driving carefully .
So adding 4 psi to the already high recomended pressure is not needed.
If you calculate it yourselfes with use of 99% acurate determined axleloads in your use ( succes with that, the most tricky part, and your responcibility) you can add 10% for reserve , and if you use maxload in the lineair calc, it still gives acceptable comfort and gripp, and max speed, wich you wont go over for even a minute 99 mph, not that hard to stay below.
Even if calculated for GAWR's it might come to in the 20s psi.
Formula for lineair calc, upto 99mph( official calc comes to slighty lower pressure) is next.
(( real axleload+10%)/( 2x maxload of tire) x reference-pressure= to use cold pressure.
Then fill that cold at about 20 degrC/ 68 degrF, and let it flow cold and warm, with ambiënt temperature.
Main goal of tiremakers,is to not overheat any part of tire-material, when driving the speed constantly, for wich deflection of tire is determined, so pressure is calculated for the load.
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If you really want to know search on google or youtube for how to do the tire chalk test. In summary, you chalk the face of the tire and watch for how the chalk wears off as you drive a little. You want even wear across the face of the tire - ie even wear of the tread.
I have found my Nissan's are about perfect at OEM recommendation. My Toyota seems a little low.
I have found my Nissan's are about perfect at OEM recommendation. My Toyota seems a little low.
The chak test assumes if right pressure chalk wears evenly from side to side of treath, and was first introduced for diagonal tires.
A radial tire ( almost only used nowadays) stays with its treathwidth on the ground within a large range of deflection ( so pressure for load) .
So pressure must be mucht to high or low, before chalk wears of on sides or centre.
Then it can be that chalk wears even, but at high speed still overheated tire-material, wich is only allowed " ZERO " times in tires use, and can lead to blowing tire or treath-separation mayby only after a year, when beginning internal cracks have builded up enaugh in time. Then other factors are blamed.
So if chalktest used, begin high ( then centrewear), and stop when chalk wears even. Dont go on lowering pressure until chak wears of on sides .
A radial tire ( almost only used nowadays) stays with its treathwidth on the ground within a large range of deflection ( so pressure for load) .
So pressure must be mucht to high or low, before chalk wears of on sides or centre.
Then it can be that chalk wears even, but at high speed still overheated tire-material, wich is only allowed " ZERO " times in tires use, and can lead to blowing tire or treath-separation mayby only after a year, when beginning internal cracks have builded up enaugh in time. Then other factors are blamed.
So if chalktest used, begin high ( then centrewear), and stop when chalk wears even. Dont go on lowering pressure until chak wears of on sides .
I run about 5-6 psi over on my car in the summer. Winter about 3-4.
The manufacturer's recommendation is a compromise figure between ride comfort, tire life, fuel economy, and safety. One may deviate slightly from the recommended pressure as one may see fit to enhance one or two of these without too much decrease in the others.
Run less in rear on FWD's or FWD based AWD's, as they are many times 60% weight in front.
Run way less on 3/4-1 tons, as they ride like bricks @ placard (solid front axles, anyway) and placard is rated for max loads.
RoadForce numbers increase with pressure, sometimes significantly, at even 10 PSI difference.
Run way less on 3/4-1 tons, as they ride like bricks @ placard (solid front axles, anyway) and placard is rated for max loads.
RoadForce numbers increase with pressure, sometimes significantly, at even 10 PSI difference.
For my regular 4 hour highway runs, I trust and employ the "Add 10% psi for sustained highway speeds" advice.
I believe the idea is the maintain the profile of the tire.
I believe the tire hitting the roadway creates a buckle in the tread, however small, which also affects the sidewalls.
The added pressure keeps the tire contact constant and eliminates the buckling (a shock wave of sorts?) having to return to round.
Yes, I dutifully reduce the pressure at trips end.
My tires wear squarely and evenly.
I believe the idea is the maintain the profile of the tire.
I believe the tire hitting the roadway creates a buckle in the tread, however small, which also affects the sidewalls.
The added pressure keeps the tire contact constant and eliminates the buckling (a shock wave of sorts?) having to return to round.
Yes, I dutifully reduce the pressure at trips end.
My tires wear squarely and evenly.
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I have two vehicles that allow me to monitor the tire pressure as I drive. Stated temps on the label are for COLD tire pressure without a heavy load, as in NOT all seats occupied by adults. In the winter's current temps I usually start with about 2 or 3 psi below. After 20 mins to half an hour they are usually reading +1 to +3 psi above stated psi. With one or two occupants in the vehicle the ride gets significantly harsher if the psi is much more than indicated.
Couple of assumptions others have suggested...
1. Significantly lower pressures can harm the tire walls, particularly at this time of year with all the potholes.
2. Higher pressure will reduce rolling resistance (better mileage) but will also accelerate the wear on the tires.
Others, chime in!
Couple of assumptions others have suggested...
1. Significantly lower pressures can harm the tire walls, particularly at this time of year with all the potholes.
2. Higher pressure will reduce rolling resistance (better mileage) but will also accelerate the wear on the tires.
Others, chime in!
The placard does not know how cold it is day to day, nor does it track the ever so slight gradual loss of PSI over time. Run 2-3 PSI over to allow for this.
There are 2 camps in how to handle cold pressure and temperature.
both camps state that cold pressure is when temperature of gascompound in tire is the same as ambiënt temperature. So at 100 degrF hot day, cold is also 100 degrF in tire.
Largest camp states that the determined needed pressure for load and speed, must be filled cold at any ambiënt temperature, be it 20 degrF or 110 degrF.
My camp states that determined needed pressure for load and speed is for cold at 20 degrC/ 68 degrF ambiënt temperature, and for other ambiënt temperatures you have to calculate it from that index-temperature to the temperature in tire, so cold the ambiënt temperature.
Nature aranges it itself , pressure rises and falls automatically with temperature-change.
Porche and BMW work in their tpms systems with this, and the term index-temperature is introduced by a mining tire company Rimex.
If the calculate right, I dont know, can be they use a rule of tumb calculation, wich would be a shame, the exact formula also fits easyly in the software.
Can also be they did not connect it to the low pressure warnings, so on cold mornings still that anoying low pressure light blinks.
Idea behind it ( camp 2) is that main goal is not to overheat any part of tire-material.
When now hot outside , the higher ( cold) temperature in tire highens up the pressure, so lesser deflected tire, so lesser heatproduction.
This compensates the lesser cooling down , because of lesser temperature differences between tire-material and in and outside tire air.
So never blead down warm or cold pressure on a hot day, then lesser cooling down, but higher heatproduction, so tire-material overheats.
When cold ambiënt temperature, the other way around, but then you are allowed to highen up for fuelsaving or riding- quality, tire-material only stays cooler then.
both camps state that cold pressure is when temperature of gascompound in tire is the same as ambiënt temperature. So at 100 degrF hot day, cold is also 100 degrF in tire.
Largest camp states that the determined needed pressure for load and speed, must be filled cold at any ambiënt temperature, be it 20 degrF or 110 degrF.
My camp states that determined needed pressure for load and speed is for cold at 20 degrC/ 68 degrF ambiënt temperature, and for other ambiënt temperatures you have to calculate it from that index-temperature to the temperature in tire, so cold the ambiënt temperature.
Nature aranges it itself , pressure rises and falls automatically with temperature-change.
Porche and BMW work in their tpms systems with this, and the term index-temperature is introduced by a mining tire company Rimex.
If the calculate right, I dont know, can be they use a rule of tumb calculation, wich would be a shame, the exact formula also fits easyly in the software.
Can also be they did not connect it to the low pressure warnings, so on cold mornings still that anoying low pressure light blinks.
Idea behind it ( camp 2) is that main goal is not to overheat any part of tire-material.
When now hot outside , the higher ( cold) temperature in tire highens up the pressure, so lesser deflected tire, so lesser heatproduction.
This compensates the lesser cooling down , because of lesser temperature differences between tire-material and in and outside tire air.
So never blead down warm or cold pressure on a hot day, then lesser cooling down, but higher heatproduction, so tire-material overheats.
When cold ambiënt temperature, the other way around, but then you are allowed to highen up for fuelsaving or riding- quality, tire-material only stays cooler then.
or if you dont want to change your tire pressure twice a day from temperature swings.
Its really not that precise.
A vehicle with 1 person in it commuting that has 30psi vs 35psi.. is not going to ruin the tires.. the same for 40psi.
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Kestas
Staff member
How many people remember the Ford Explorer/Firestone debacle? Many things came to light from that fiasco. For one, we learned that a low tire pressure was recommended to reduce the vehicle's center of gravity. Many other manufacturers recommend a pressure for best ride comfort, not for what is best for the tire and vehicle.
I run 35 psi on all tires of all cars I own. I often load the car for trips. Plus, I have a psi cushion in case a tire starts to leak. Combined with regular rotations, my tires life a long, healthy life.
I run 35 psi on all tires of all cars I own. I often load the car for trips. Plus, I have a psi cushion in case a tire starts to leak. Combined with regular rotations, my tires life a long, healthy life.
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- Sep 2, 2016
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Thanks for sharing. The "higher PSI for more MPG" is often repeated online without any substantive evidence.
There are many factors that the manufacturer considers when specifying tire pressures that are not immediately apparent to us. As an example, BMW has different specified tire pressures based on vehicle trim (eg Sport suspension), tire size/load capacity and operating conditions (maximum load, maximum speed, etc). I've tried to decipher it for patterns and one thing I noticed was that they also tune for less understeer on sportier trims.
Regardless, I'm not smarter than their engineers so I simply run what is specified for my usage.
Had Yokohama YK-CTX's installed on my Mazda CX-5 at DT in Yuma, AZ last week. They fill the tires they install 3PSI above the Mazda pressure of 34 PSI. the reason is the cold night time desert temps. They said they are more concerned with people running their tires at lower pressures, than higher pressures. So after the drive back to Tucson in 95 degree heat only increased the tire pressure to 39 PSI.