The calculated distance (air line) between Spiez and La Chaux-de-Fonds is approximately 50 miles respectively 80 kilometers.
By car or train, the actual journey to La Chaux-de-Fonds is certainly longer, as only the direct route (as the crow flies) between Spiez and La Chaux-de-Fonds has been calculated here.
Air line (approximately)
80 kilometers
43 nautical miles