As it affect fuel consumption,performance, sluggishness, cornering etc. Important is the outer diameter. Someone wrote a nice calculator. HTHs you at some point. :-)

 

Which also bring me to this : From a tech point of view and using the correct tools, what should be the correct tyre pressure for a given tyre size, terrian and temperature ( as the air inside a tyrer will expand with heat ) ? I think 28 psi for regular is too standard and everyone here seems to go for it.