The FIRE engine should run around the 90 degrees mark most of the time. My cinq sporting would not budge from 50 degrees when driving around, but when I changed the thermostat it would sit between 85 and 95 from then on. Anyway, underheating is not a huge problem, just means your engine may not be working at max effeciency. Having said that, I never experienced difference in fuel economy in mine.
Good luck!
Phil