The reference distance is the distance between the front reference plane and the rear reference plane. It is NOT necessarily the wheelbase.

The front reference plane is where the longitudinal coordinates of all the front suspension points are measured from. This plane can be anywhere the user wants to measure the points from. (E.g. front spoiler, vehicle origin in CAD, wheel center line etc.). The distance from the center of the front contact patches to the front reference plane is called Longitudinal Offset this distance is defined in the wheel geometry section of a design.

The rear reference plane is where the longitudinal coordinates of all the rear suspension points are measured from. This plane can be anywhere the user wants to measure the points from. (E.g. front spoiler, vehicle origin in CAD system, wheel center line etc.). The distance from the center of the rear contact patches to the rear reference plane is called Longitudinal. Offset this distance is defined in the wheel geometry section of a design.

OptimumK uses reference distance instead of defining a wheelbase because some oval track race cars have different wheelbases on the left and right hand sides of the car.