Originally Posted by
Bikearound
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.
I wasn't speaking of lowering pressure for mud. Nice resource there:
Conclusion
Tire pressures that correspond to 15% tire drop will optimize
your bike’s performance and comfort on average road surfaces. On
very rough roads or unpaved roads, it may be useful to reduce the
pressure. On very smooth roads, increasing the pressure slightly
may improve the performance of your bike