The gap between the pinion and spur gears need to be set correctly for the them to work efficiently and to prevent premature wear.  Too tight of a mesh can result in overheating and reduction of the vehicles overall speed due to the increased friction and/or premature bearing failure due to the increased load.  Too loose of a mesh will lead to premature gear failure as there won't be enough strength in the area of the teeth that are contacting each other.

Every motor is capable of being adjusted for setting of proper gear mesh.  Electric motors are generally on a slide or swinging arc while nitro motors are on a side to side sliding cradle to move the motor closer or farther away from the spur gear.  Watch the animation to see just how the motor moves to and from the spur gear to achieve the desired distance to set proper gear mesh.

Here the gear is set too far from the spur gear.  Since only the tips of the teeth will make contact, this will result premature gear failure.

Here the gear is set too close to the spur gear.  Premature gear failure will occur due to the excess friction.  Other problems such as over heating and premature bearing failure may also occur due to the stresses that the motor will encounter from such a setting.

The easiest way to achieve proper gear mesh is to place a strip of paper between two gears and push them together, trapping the paper between the gears.

Spin the gears and the paper will be released.  Watch for the crinkle as that will tell you that you have good gear mesh.  If the crinkles are too smooth, you need to tighten the mesh.  Tears in the paper will tell you that the mesh is too tight.

Here the mesh is set just right.  Good contact with the gear teeth and no binding.  After trying this a few times, you will no longer need this trick and will be setting your gear mesh simply by eyeing the gears.