F100, if you modified the dial from the shipped setting, then you might be running a lower resistance than expected. The shipped setting is 82-83 degrees F. I have seen a few of the tuner plugs visually between 40 and 50 is 82-83 ohms. If you want to get an actual resistance value you can remove the tuner plug from the vehicle, get two tiny paperclips and measure the resistance with a multimeter. I bet the 50 setting will work well for you. For every 10 ohms of resistance, 2-3F is reduced from the ECU gauge, at least on freeway operating temps from my testing.
I did some idle testing vs freeway driving tonight. I removed my tuner harness to ensure a completely stock test was done with no extra variables.
10 minute warmup, 5 mile drive for 55mph-65 in D3 to bypass VCM most of the time and get engine up to temperature. My ody has a 180 thermostat Used a veepeak scanner. Engine Coolant Temp sensor on torque w/ dial mode.
Average temperature at 55-60mph - 176F - freeway type driving
Peak temperature after coming to a stop for 3-4 minutes - 206F. This was with the AC Off. I normally run auto but wanted to see what would happen. I was pretty amazed it got this high. Even in the winter.
Peak temperature after coming to a stop for 3-4 minutes w/ AC-On - 190F. This is with the auto button enabled and AC On set to 70 degrees.
I noticed the temperature would more rapidly increase w/ the AC off. Within a few minutes temps hit 194 F w/ it off.
So it seems that leaving the vehicle in auto mode for the climate control may stabilize the coolant temperatures because the extra fan kicks on when the AC is engaged. I normally leave the AC on auto w/ the climate setting.
At least on my van, that's a considerable temp swing. I'll likely consider starting to run setting 50 myself (some 98-102 ohms) and see how it goes on the tuner now-on with things starting to warm up.
So the 82 ohm setting is normally shaving some 13-16 F off the freeway operating temperature to get down to 161-163F. The math says overall that's up to 1 degree per 5 ohms. It's not perfectly linear however because a 10 ohm change
when at operating temperature on the freeway only seems to shift some 2-3F for my vehicle. At 5 degrees per ohm, to overcome a 205F temperature and knock this down to 167 F, that's 190 ohms of resistance. However, at 3F per ohm,
this would be 114 ohms of resistance required, which is at least more reasonable. With AC on in the summertime, the coolant temps will likely require a 115-120+ ohms resistance to keep the eco light off. Ambients here are up to 112-113F.
At least now you have the power to tune - in terms of having the eco light on occasionally or have it off all the time. Just be careful because too high of a setting deviated from normal operating temperature can generate a CEL on some models.
Hope this info helps.