what is the logic behind underinflation?

[STSinNYC]

The MINIMUM cold inflation number is the figure on the manufacturer's label. You should never let your tires go below that number, for safety, warranty etc. The MAXIMUM cold inflation number is the figure on the tire. You should never let your tires go above that number for the same reasons. Where you choose to inflate your times between those numbers is a matter of preference, ride comfort vs. longer wear, better handling, better fuel economy. For our 03 Accord, Honda's label says 30 PSI F & R and the cold max is 44. We keep them at 40, the level recommended by a family member who has been in the tire business for 25 years and knows a great deal about this question.

 

[Tornado Red]

To answer the question in the heading --what is the logic behind underinflation? -- the answer is that if you are stuck in deep sand, in the Sahara Desert during the Dakar Rally for example, you will want to underinflate until you reach solid ground.

Then reinflate to normal pressure, whatever that might be.

 

[Drew99GT]

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The MINIMUM cold inflation number is the figure on the manufacturer's label.

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On my Toyota, it indicates the cold pressure on the door placard is for maximum vehicle weight and capacity, so you could easily go down a psi or 2 and be at the same point load wise with an unloaded car.

 

[Blokey]

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I go by tread wear pattern. Door placard says 35 all around. Im running 40frt/30rear.

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That's exactly what I do with our two nose-heavy FWD cars. I've found that running higher pressure up front and lower pressure in the rear yields excellent treadlife plus I'm content with the ride and fuel economy. Our one 50:50 weight balance RWD car gets 35 psi all around.