Originally Posted by
LeeG
Tire pressure could make up a big part of the difference with one tire covering road and dirt but you have to specify total load on the tires.
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Here is an interesting piece on tire pressure:
http://www.vintagebicyclepress.com/images/TireDrop.pdf
I don't think you can regulate the tire pressure to make it better for one surface and then another. Every tire is designed to work best at a given tire pressure and then fine tuned for the weight being carried. To lower the pressure in a tire to make it wider and theoretically better in mud I think is somewhat of a myth.