Assuming this is CFS2, FS2002 or FS2004 change the appropriate [contact_points] entries in Aircraft.cfg.

Example from the default Corsair.

[contact_points]
     //0  Class
     //1  Longitudinal Position        (feet)
     //2  Lateral Position             (feet)
     //3  Vertical Position            (feet)
     //4  Impact Damage Threshold      (Feet Per Minute)
     //5  Brake Map                      (0=None, 1=Left, 2=Right)
     //6  Wheel Radius               (feet)
     //7  Steer Angle                     (degrees)
     //8  Static Compression           (feet)  (0 if rigid)
     //9  Max/Static Compression Ratio
     //10 Damping Ratio               (0=Undamped, 1=Critically Damped)
     //11 Extension Time               (seconds)
     //12 Retraction Time               (seconds)
     //13 Sound Type
     //14 Airspeed limit for retraction (KIAS)
     //15 Airspeed that gear gets damage at (KIAS)

point.0= 1, -18.00,   0.00, -3.35, 3200.0, 0, 0.50,180.0, 0.25, 2.5, 0.90, 1.0, 4.00, 0, 0, 200            //tail wheel
point.1= 1,   1.80,  -5.93, -7.65, 2200.0, 1, 1.35,  0.0, 0.54, 2.5, 0.76, 5.5, 4.50, 2, 0, 200     //left wheel
point.2= 1,   1.80,   5.93, -7.65, 2200.0, 2, 1.35,  0.0, 0.54, 2.5, 0.76, 5.5, 4.50, 3, 0, 200     //right wheel