According to this American Public Trasportation Association study. The APTA is the most active nation-wide transit advocacy organization. The study calculates the cost of gas compared to transit costs for a person who drives 15,000 miles a year, pays $3.909 per gallon and gets 23.3 miles per gallon. How the transit pass is calculated is not mentioned in the post.
Seattle comes in fourth on the list with $8413 in savings per year, behind only Honolulu, Las Vegas and San Francisco. But of course, we shouldn’t build light rail because that would just let more people save $8413 per year.