Schwalbe on resistance, scroll down for the graph.
http://www.schwalbetires.com/tech_in...ing_resistance

Bought a new MTB 3 weeks ago. It came with a 100mm, +7 degree stem.
Had a 90mm, 0 degree stem here and it brought the bars to just below the seat. Much better for pedal power and my old lower back.
I will be ordering a 50mm, -6 degree stem. This bike has 27" wide bars and they open my chest for easier breathing.

A simple stem app. someone here @ BF posted.
http://alex.phred.org/stemchart/