On rolling hills, there is a momentum factor. You pick up a lot of speed going downhill, and your momentum carries you quite a ways up the next. If the uphills are not too long, sometimes very little effort is required. When climbing longer hills (we live in Colorado), my experience had been that our climbing speed is about the average between my solo climbing speed and stoker solo climbing speed. Basically, the power is the average of the two riders. When we first started riding the tandem, I tried to climb at the same speeds I was used to solo. All I did was blow myself up! Learned to keep and eye on the heart rate monitor to keep from redlining every time we went uphill!