Complex sentence: The vehicles were most popular in cities, where the relatively smooth roads made the superiority in smoothness and silence of the electric motor over the gasoline engine obvious, and where electrical supply for recharging was easily found within the runabout's 40 mile (64km) range.
Split into simpler sentences: The vehicles were most popular in cities, where relatively smooth roads made the electric motor, with its smoothness and silence, appear superior over the gasoline engine. It helped in urban areas that electrical supply for recharging was easily found within the runabout's 40 mile (64km) range.