As previously suggested in this thread, you should look into how hot your engine can get before issues pop up. I've read that my GM 6.5 Turbo Diesel can't get past 205 F (or is it 210?) or else it should start to have problems.
You might also want to check this thread out:
https://ecomodder.com/forum/showthre...mpg-37356.html
The thread's not specifically about thermostats, but it's about Smokey Yunick's Hot Engine. However, there are some mentions about a hotter thermostat (203 or 205) raising the F.E. just a little bit.
Also, keep in mind that a hotter thermostat can cause your oil to break down sooner. If you're maintaining your car with regular oil changes with Full Synthetic Oil, though, this shouldn't be as big of a problem. And while I'm on the subject, if you're not already doing so, you might want to consider some of the best oil filters on the market: FRAM Ultra Synthetic, or Amsoil EaO. Out of all common (or fairly common) filters on the market (not counting "bypass" filters,) these two will capture the most dirt in the oil and keep your engine in better shape. The FRAM is even cheaper than glorified brands, such as Mobil-1.
And if anyone's concerned about me recommending a FRAM filter? I recommended their most expensive filter, not the cheap one with a rubber band or whatever it is.