Inspector-in-charge of Harirampur police station in South Dinazpur district, Nilesh Ghosh, and Sarwar Hossain, a sales manager of a private insurance firm, were killed when their car hit the gas tanker from behind.
Police suspect the car's brakes had failed, leading to the mishap.
The victims were on their way to Raiganj in North Dinajpur district from Siliguri when the incident occurred, police said.