please do tell us if your sales advisor responds to that.
He just replied. He said "Regarding wheel size affecting the range, the Official EPA range up to 516mi for the GT is based on the 19inch Range; as you go up an inch to 20inch wheels, you will lose roughly 23 miles of range; with the 21inch wheels, you will lose about 47miles of range, depending on driving condition and style. As the wheel gets larger, there is more ground to cover, which gives you better handling and feel of the road. Most of my customers felt the road much more on the 21inch vs. the 19inchs, where they felt less impact when going over rough roads with many holes or cracks."
This and the chart posted by Adnillien above are super helpful.
In my mind the tradeoffs are:
Handling - 21 best to 19 least (less sidewall = less flex so more crisp handling)
Road smoothness - 19 best to 21 worst (more sidewall = more flex to absorb road variations, also less road noise)
Mileage - 19 best, 5% penalty for 20, 10% penalty for 21 (more wheel diameter = more rotational inertia, more air resistance)
Based on this, and the look of the wheel choices, I'm thinking 20's.... I don't need all season tires, that could definitely change your decision.