StangGT1995 is right about the accuracy of these gauges. And the accuracy gets even worse as the car ages. Over time corrosion can form on many hidden grounding points. Think of bare metal grounds that accumulate slight amounts of rust. Rust creates resistance and affects sesnsitve circuits like gauges. Over time, the readings can be up to 15 - 20 degress off on a temp gauge. Bad grounds and excessive current draw can also cause negative return voltages. You can find the amount of negative return voltage by using a voltmeter and measuring by connecting to two separate grounds. Take the negative battery terminal and the ground connection near the passenger side headlamp. If the grounding between the two is bad, you can have voltages of up to 1 volt. Cleaning ground points can help alleviate some of this. Now, to see the affect of bad grounding on a gauge, drive your car around till it's well into normal operating temperature (near the middle). Now, idle it someplace safe and note the gauge reading. Also note if the cooling fan is on or off. If it's off, read the gauge then flip the vent switch to any of the positions that engage the compressor (eg.. Defrost). What you want to do is flip the switch on for 2 seconds and and then flip it back off. Just long enough to cause the fan to kick in but not to engage the compressor clutch. Keep watching your temp gauge the whole time your doing this. When the fan kicks on, the temp needle can nudge to the right a little. This is from the high current draw of the fan creating a negative voltage return, and in the cases of corrosion. Sometimes the effect of this can be so great that the gauge needle can move up to the amount of one full letter in "NORMAL" . Where it normally would be on "R" ,the fan kicking on nudged it near the "M". Now, while the fan is still on, press the rear window defroster button (if you have it) and note the temp needle. Same thing, but combined with the fan on the needle moves even more. After the fan shuts off and the load drops, so does the needle.
My 96 has been doing this throught the past 3 years. It's only been getting slightly worse over time. This may or may not be relevant to your issue but I thought it would give some insight on the possible cause of your high temp position.
Has your stang always had AC equipped? Or did you do a fresh install and mod a vehicle that didn't ever have AC. Just curious, though I highly doubt they make them that way these days. AC is a factory no-charge option.
As for T-stats, 192 degree is the norm. Going to a lower degree t-stat such as 160 may decrease your fuel economy (not sure if you care about that hehe) . I have removed mine indefinately because of cooling system pressure issues. It's not killing my fuel efficiency much as I live in Florida and it gets hot here all the time.
On a last note, Idling with your AC on for extended periods in hot weather can greatly increase the negative return voltage effect. The alternator gets very hot and voltages are reduced from heat stressing the diodes and regulator circuit. I used to idle in my car on my lunch breaks at work very often. After awhile, the temp gauge would get well into the "M" or a pinch past that. Sitting there for extended periods doesn't help heat dissipation any bit. It can't cool off very well and do no more than blow hot air at things that are already hot.
