Wheelbase is measured from center front wheel to center rear wheel (front wheels pointing straight ahead!).

On some vehicles the wheelbase on the right differs from the one on the left. If this is an engineering feature (usually on cars with torsion bar suspension in east/west orientation) than average the two numbers.

On vehicles with different wheel diameters front vs. rear, the wheel base is measured between the projections of the wheel centers on the ground. Not between the physical wheel centers.