Originally Posted by
maltess
So what if would happen if I get a MTB and change the velocity gear to a road bike, I mean bigger crank etc.
You would be able to go faster assuming you have no compatibility issues and the leg strength.
If you primarily ride on the road, the bike might be more comfortable (geometry wise, not material wise as in steel in more comfy), heavier, studier, and less aerodynamic than a regular road bike.