The economic appeal of shipping US-refined gasoline on cheaper foreign vessels has been waning in recent months, after the US eased sanctions on Venezuela, a move that triggered an increase in regional freight prices. Foreign ships, which were nearly $4 a barrel cheaper than US-flagged ones in the past year, are now barely $1 cheaper, data from Argus Media show. If freight costs continue to rise, shipments of US gasoline could become too expensive to compete with supplies from South Korea or India.