What sort of tyres on your "mountain bike"? Full suspension, or just a hard tail?
The mountain bikes both have front suspension forks and hard tail, and both run 1.9" in the back and 2.1" in the front. They might have done better if I'd lowered the pressure. Had them at 50# for our usual city riding.
The fatties have no suspension beyond what the big tires provide. We were running them on the hard side, around 10#.