Tiny or micro vehicles require significant adjustment in ACS default script values, as very small vehicles provide very little resistance to Second Life physics, and need only small forces applied to steering and gears. Some additional steering power forces will be overkill and should be turned off.
You will want to lower your gear values in the root settings as well as steering power.
You probably need to set to zero your VelMult and Turn Bonus from the > Options,> Controls, > Steering menu. Or you can directly turn those off in the Root Settings by setting your Init_Turn_Bonus and Init_VelMult to 0.
If that is not enough, lower the Z value for VEHICLE_ANGULAR_FRICTION_TIMESCALE on line 123 of the Root Settings.
Even with appropriate steering and control settings, very small vehicles may do things such as easily tip over under the weight of the avatar. You may ultimately need to resort to using invisible prims to make "training wheels" to prevent tipping over.
Tiny avatars may require special custom tiny animations, examples of which are not provided with ACS. I'm not really an expert in tiny animations, but several builders in the Owners Group are and can steer you in the right direction.
Smaller vehicles, like tricycles or whatever may be more likely to have issues with animation bounding boxes, so custom animations may be needed.