Practically, especially on a highway, that is true, but I wonder, theoretically, what your fuel economy is at 10 MPH in second gear, letting the engine idle without touching the gas, or letting it idle in third gear without touching the gas.
Our Prius at work has a dashboard display setting that will show your current fuel economy. Idling around campus, I’ve seen it go up to 100 MPG on battery-only power. I’m sure your Yaris isn’t a hybrid, and I know this is all academic, but are you sure you can’t go slow enough to make your Yaris get 80 MPG? I’m not.