The average Japanese household has more than $86,000 in savings, up 16.6 percent from a year earlier, according to a study published today. It was the biggest increase in savings since 1973, when the average rose 28 percent from the year before, according to the national survey of more than 4,000 families. The annual survey was conducted in June and July by the Central Council for Savings Information, a non-profit group of Japanese financial institutions and scholars. The survey said the average rose to $86,700 because of an increase in earned income from a strong domestic economy. In September, Japan entered its 46th straight month of economic expansion.