I have a 2021 ZL1 with about 1800 miles. I get about 12-13 mpg, mostly city driving. The best I have ever got was 20mpg on the highway cruising in 10th gear and 65mph. I am not sure how you can average 21mpg combined in this car, maybe a very, very light foot.
and yes once you start putting your foot in it you will probably get 6-8mpg.
Some going all out on the track get 3-4mpg.
Supercharger, more air, more fuel.