New Delhi, March 18 -- THE COASTAL Chinese city of Zhuhai is linked to Hong Kong by a showy piece of infrastructure: a 55km bridge and tunnel, the largest sea crossing of its kind. Some Hong Kongers use it to visit Chimelong Ocean Kingdom, a theme park featuring a whale shark, rollercoasters and a hotel shaped like a spaceship. Others are motorists with a more mundane purpose. They travel to Zhuhai to fill up their tanks with petrol, available at a big enough discount to make the drive worthwhile.

The mainland's petrol-price formula smooths out the international market's ups and downs. As such, it is one of the ways China is shielding citizens from the effects of the war in Iran, which has trapped oil tankers on either side of the Strait...