[aerohead]

You've probably shot yourself in the foot. The active shutters are a known quantity. And their effect varies from vehicle to vehicle.
The Chevy Malibu realizes only a 7-count drag reduction, and an owner's ability to experience a difference in mpg would be statistically impossible.
The Lightyear Zero experiences a 6.6% drag reduction, which 'WOULD' show at the charger.
You've helped ruin the ability of the engine to come up to operating temperature in cold weather, a great reason to have the shutters. And your urban driving is 'TOO SLOW' to discern any change, aerodynamically.
Whether they open on the highway or not will depend on ambient outdoor temperature and engine load.
If you live in Furnace Creek, Death Valley, they'll be open a lot more than if you lived in Minot, N. Dakota.