You can’t compare deltas in instantaneous MPG because that is scaling the difference in fuel use for AC on or off based on the inverse proportion of fuel already being used to propel the vehicle forward. You need to look at gallons per hour changes when the AC is turned on to get an apples to apples comparison of the options.
Assuming an average speed of 60MPH, 23MPG=2.609 Gallons per Hour and 21.5MPG=2.791 Gallons per Hour, so a delta of 0.182 Gallons per Hour. Once again assuming an average speed of 60MPH, 200 MPG (Average of 190 and 210)=0.30 Gallons per Hour and 125MPG=0.48 Gallons per Hour, so a delta of 0.180 Gallons per Hour.
So the difference between the options in Gallons per Hour is 0.002, which is well within measurement noise for numbers given to very few significant digits, making the difference for you between the options virtually identical. So assuming you run the AC for the same amount of time in terms of minutes out of the hour for the two options, the good news is it really doesn’t matter very much for your car, so feel free to run it whenever you wish.
