Bad thermostat = bad MPG?

[Doubt Incarnate]

september is tapping me out, my explorer pulled double duty and while my brothers car was down and its used up my gas budget already. before it gets cold out i'm going to hit it with a tune up, i know the thermostat is bad, and in the 90+degrees of summer i didnt feel it necesary to replace it since the engine was running cool. the winter is different though, i'm replacing it. major question is with the engine runnig cool did i burn out my gas faster? gets around 18-20mpg, more highway than mixed.

 

[SoNic67]

For a V8 from '99, that mpg is as good as it gets.
Sure, thermostat should be functional in winter, the engine PCM won't go in "closed loop" without reaching proper temperature. And that translates in bad mpg, possible damage to catalitic convertors...