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Tinsoldier wrote
the manufacturer sticker posted in door is rated for comfort and performance.
this is not true, the sticker gives a calculated lowest pressure needed for the GAWR ( gross axle weight rating) if equally devided R/L, so what is maximally allowed on the axle. They are not interested in comfort and performance only that the tires wont blow.
Then this is calculated with the universal formula used by all tire makers over the world , but with a different power in it then European tyre-makers use, wich gives lower pressure for the load then the Eur calculation.
Formula is for calculating loadcapacity for pressure La/Lmax= (Pa/Pr)^x or the other way around presure needed for load Pa/Pr= (La/Lmax)^1/x.
La= actual load or loadcapacity
Lmax= given maximum load of tire
Pa= actual pressure or pressure needed for La
Pr= Pressure needed for maximum load up to max speed of tire or if lower 99m/h
X= power used and this is the tricky thing. Power 2 is squaire and power 1/2=root. Eur uses X 0.8 for all kind of tires , and USA used 0.5 up to 2005 for P-tires and 0.7 still for LT ( from 2006 x= 0.8 for P and XL). There are still many old calculations and lower X means higher loadcapacity for the same pressure, or lower pressure for the same load.
So calculated with old american way gives a lot of comfort but sometimes to low pressure so heat generation and tire failure.
A normal car use does not give the total GAWR on the axles , only when towing or fully loaded. So for normal car use ( and thats also lighttruck 95% of use) this gives already reserve , but for fully loaded cars or when towing or motorhomes or trailers , it will be to low. Motorhomes and trailers are often unequally loaded R/L so need pressure for the highest load on tire.
And then even a reserve up to 20% on the lowert load, is usefull and still gives no terrible bumping.
This all highens up the needed pressure for special conditions like towing. Higher even then the pressure advices give.
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and see for yourself. r
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