Originally Posted by
terrymorse
Once upon a time, I did a long climb twice: once on a road bike, then on a mountain bike. I kept my heart rate in the same range to maintain the same level of effort, and I did the two climbs on different days. The mountain bike was 15% slower, even though the MT bike+rider mass was only 3% more than the road bike+rider mass.
Mountain bikes are slow.
I'm assuming you didn't have knobby tires on the mountain bike. Same tire pressure? Same type of tires?
It's certainly true that mountain bikes put riders in a less aerodynamic position for road bikes, but for climbing, this should matter less than for flats.