The Fuel Saver Device (FSD) is an advertisement in the March 2014 issue of Family Motor Coaching magazine, page 99. The ad says that tests on a 435-horsepower Cummins diesel motorhome resulted in an increase in mileage from 5.7 mpg to 8.3 mpg. That sounds good, if it is correct. Looking for someone who is using the FSD. Thanks. Mike