I believe the ideal air pressure is just low enough that with the full load the contact patch is still reasonably flat left to right. If you have any less air than that then the center of the tread will pucker up and give you less traction.

That, and not spinning the wheels in the tires is what you look for when letting air out for maximum traction.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-