Water use
A family of four uses roughly 9,000 gallons a year at 1.28, against about 11,500 at 1.6 — a saving of around 2,500 gallons.
Most water authority rebate programmes require 1.28 or less, so these models generally qualify.
Why 1.28 is the default
Federal law caps toilets at 1.6 gallons, but the EPA's WaterSense programme requires 1.28 or less, and several states have adopted that as their legal maximum.
TOTO's current flushing systems were engineered around this volume, so there is no performance trade-off against an older 1.6 gallon model.
Does it flush as well as 1.6?
Yes. The clogging reputation of low-flow toilets comes from the first generation in the 1990s, which were simply restricted. Modern bowls are designed around the volume.