No disagreement with James on the mountain bike tires, and the mountain bike would probably be a lot easier to ride over the grass surface than my hybrid was.
But the issue would still be there. A hard surface is a hard surface, and, unless you're riding on very soft asphalt, chances are that your tire will deform the same way on any hard surface. Even a wide tire will deform differently on hard earth than it will on soft earth or grass. It's impossible to calibrate on exactly the same surface as you measure.