The calculated distance (air line) between Saint-Étienne-de-Montluc and Rennes is approximately 58 miles respectively 93 kilometers.
By car or train, the actual journey to Rennes is certainly longer, as only the direct route (as the crow flies) between Saint-Étienne-de-Montluc and Rennes has been calculated here.
Air line (approximately)
93 kilometers
50 nautical miles