The calculated distance (air line) between Saint-Germain-lès-Arpajon and Rouen is approximately 79 miles respectively 127 kilometers.
By car or train, the actual journey to Rouen is certainly longer, as only the direct route (as the crow flies) between Saint-Germain-lès-Arpajon and Rouen has been calculated here.
Air line (approximately)
127 kilometers
68 nautical miles