FRANKFURT (Reuters) - Fiat Chrysler and Peugeot owner PSA's pledge not to close factories if they merge is likely to come under heavy strain as the combined group would have spare production capacity of almost six million vehicles in a slowing autos market. The companies last week unveiled plans to create a $50 billion (£39 billion) group that would leapfrog Hyundai, General Motors, Ford and Honda to become the world's No.4 automaker, based on their combined 8.7 million vehicles sold last..