Unused:
I know Sonic Drive-Ins have shakes worth like 2k calories.
Highest I've seen from Sonic is around 1,500 from what I remember, but they're also the largest size at 32 fl. oz.
The most calorie dense option I'm aware of are certain varieties of shakes from Steak & Shake. Although the varieties with the most calories are 880-980 calories, if you order one you will notice they're 16 fl. oz. which makes them more calorie dense.
If you order two of them to match the volume, there will be more calories.
