I am re-blogging a post by FLO Cycling on determining the optimal tire pressure for wider tires. The analysis is based on maintaining a constant tire casing tension, which requires lower tire pressure at the tire gets wider. The original post is located at the URL:
http://flocycling.blogspot.com/2014/11/flo-cycling-why-do-you-use-less-tire.html
For the quick summary:
Tire Dia (mm) Starting Pressure (psi)
20 120
23 104
25 96
28 86