The effect of conformational relaxation on the quantum dynamics of the hydrogen exchange tunneling is studied in the D-2h subspace of formic acid dimer. The fully coupled quantum dynamics in up to six dimensions are derived for potential energy hypersurfaces interpolated directly from hybrid density functional calculations with and without geometry relaxation. For a calculated electronic barrier height of 35.0 kJ/mol the vibrational ground state shows a tunneling splitting of 0.0013 cm(-1). The results support the vibrational assignment of Madeja and Havenith [J. Chem. Phys. 2002, 117, 7162-7168]. Fully coupled ro-vibrational calculations demonstrate the compatibility of experimentally observed inertia defects with in-plane hydrogen exchange tunneling dynamics in formic acid dimer.