The calculated distance (air line) between Sihanoukville and Kampong Thom is approximately 172 miles respectively 277 kilometers.
By car or train, the actual journey to Kampong Thom is certainly longer, as only the direct route (as the crow flies) between Sihanoukville and Kampong Thom has been calculated here.
Air line (approximately)
277 kilometers
149 nautical miles