BHarvey -
Listen to Daox. It's different strokes for different folks because the drivetrains respond differently. From what I have read about my Saturn S-Series, IAT above 180 degrees F will cause drivability problems.
For OBDII gauges that derive MPG from other parameters, I would make the claim that they are all using a form of the equation defined in this post :
http://ecomodder.com/forum/187883-post20.html
You will notice from reading the post that the IAT is part of the calculation when you need to create a pseudo-MAF (Mass AirFlow) sensor. Most cars have MAP (Manifold Absolute Pressure) sensors instead of MAF sensors.
CarloSW2