Yea sounds like the thermostat could use a replacing because the gauge should read in the middle and stay there regardless.
Right now I get about 18mpg only because my rear 02 sensor is going out and needs replacing. When everything is mechanically sound, I get about 21mpg tops.
The truck sounds ok, I've experimented with different kinds of mufflers and I found my Flowmaster is as good as it's going to get. It will get your attention for sure but it's not obnoxious. 4bangers are difficult to get a good sound from them. I would reccommend leaving the cat converter alone. Run a Flowmaster 40 or 50 series(depending how loud you want, 40 is louder) single in pipe and duals out the back.
AS far as 180* t-stats, that's the lowest you want to go with a computer controlled engine. Me personally, I've never tried a 180 t-stat so i can't say for sure. I just run a regular 192* OEM stat.
Let me point out though, I don't know if that particular Ranger is setup like mine but on the t-stat housing. Inside, you'll find a t-stat with 2 clips on the side of it and it just slides into place in the housing. I've replaced my t-stat 2 times in the 4 years I've owned my truck and only a factory thermostat will fit due to those clips. Aftermarket ones won't fit. Stupid but that's the way it is unfortunately. Good luck and hope this helps! -Mike-
Daily Ride: 98 Ranger, 2.5 Auto, K&N CAI, Flow 40 exhaust, reg cab flareside, grey metallic, 235/70R15 Bridgestone A/T's
Other Ride: 05 Harley Electraglide, tc88, Screamin Eagle stage 1 kit. Chrome lowers, nastalgic bag guards, black cherry, 1965 Electraglide Emblems