The ideal operating temperature for automatic transmission fluid in these newer Fords is 170 degrees, with peaks to 190 or so. It needs to reach at least 125 to do it's best job.

Ideal engine coolant temperature is around 200 degrees. Some like it as high as 210 even, and in many factory electric cooling fan setups (which these vehicles don't have), the high speed fan doesn't even come on until 228 degrees! It's at about 200 degrees that we see maximum power and best mileage, as a loose general "rule."

The ECU also needs to see a certain coolant temperature (usually 190 degrees) as well as the O2 sensors need to see certain temperatures (which range from roughly 450-600 degrees) to go closed-loop, which is when the ECU actually "listens" to the O2 sensor fedback for air/fuel ratio adjustment on the fly. This is when you get your best mileage, as the vast majority of driving is done on part-throttle, at less than heavy to full-throttle. So if it's going closed-loop late, or not nearly as often or for as long as you should, then mpg will certainly drop.

Yes, a too-low coolant temp. can cause problems like that, as it will cause the ECU to not go closed-loop as soon as it should, and in some cases, not at all. The factory thermostat **usually** doesn't open until higher than the temperature you're seeing, they're usually in the 190-194 degree range. I do need to point out that I don't have the exact temp for your model year memorized or a service manual in front of me at the moment, so your ECU might be able to go closed-loop below 190, but usually they require about 190 degrees of coolant temperature. So yes, it does sound like you could be a little too cool.

To get to the heart of the issue, when the motor is cooler, the ECU will automatically add in more timing, it will do that with cooler IAT's (inlet air temperature) as well as lower coolant temperatures, and that along with the Superchip could potentially be enough to push it into detonation, and will not help mileage.

It sounds like we should go over a number of things here to properly troubleshoot this. For example, it would be a good idea to verify that coolant temp reading with a scan tool, and read the coolant temp that the ECU is actually seeing. If it really is below 190 degrees, then get that t-stat changed to bring the coolant temperature up, and that be all it takes to solve the problem, of low mpg as well as the detonation issue, we could get "lucky" here and that be all there is to it, only time will tell.

Very interesting situation we have here, actually, as it will take a bit of troubleshooting to find the actual culprit. However, that will be extremely simple. Just connect a scan tool to your OBD-II port (under the dash to the right of the driver's knee) and verify the coolant tempterature being seen by the system to double-check the accuracy of your new coolant temp. gauge. If it's below 190, change out the t-stat, once coolant temps are routinely above 190 that should take care of the mpg issue, and may quell the detonation as well due to the ECU not adding in more timing from the lower coolant temp. Maybe. I don't know if for your driving and local fuel quality the mpg you're seeing may be normal. We'll just have to see what happens as you do each step of troubleshooting, and as you check mpg results over time in addition to the detonation issue; luckily this is a very easy project, all it will take is a few minutes with a scan tool to verify temps seen by the system, and then a t-stat change if it's really running that cool.