Internal rotation is a fundamental motion of end-standing methyl groups that provides important insight on the molecular physics of isolated molecules. The barrier heights of such large amplitude motions are highly sensitive to their molecular and electronic environment. Until now, it is still not possible to accurately determine barrier heights using quantum chemical calculations. To probe for the effect of molecular conformations on the barrier heights of substituted furan rings, the molecular jet Fourier transform microwave spectrum of 5-methyl furfural was recorded in the frequency range from 2 to 40 GHz. Quantum chemical calculations yielded two conformers with a trans and a cis orientation of the formyl group, which were both observed in the experimental spectrum. Torsional splittings due to the internal rotation of the methyl group were resolved and analyzed. The experimental spectra is reproduced with standard deviations close to the experimental accuracy, yielding sets of highly accurate rotational and internal rotation parameters. The results, especially the V3 potentials, are compared to quantum chemical calculations and discussed within the scope of the current literature of other methyl substituted furans, where the methyl group is in close proximity of the furan oxygen atom. Our work provides an accurate evaluation of the different case studies and highlights the bottle necks and future options of current available theoretical techniques.