Optimal MPG for '05 ES330?

"Optimal" MPG is probably the speed at which the transmission shifts into

5th gear; my guess is somewhere around 35 or 40 MPH on level ground so the transmission stays in 5th gear lockup.

At higher speeds, fuel economy will probably go down.

At 0 MPH, you get 0 MPG ;-)

Assuming the engine running. :-)

Just to add to the general discussion -- the most fuel economic speed for most internal combustion engines is where the torque curve peaks. When you put an engine in a car, then you have to take air resistance into account, which is more or less proportional to the square of the car's speed. So in a car, then engine will be most fuel efficient at a lower speed than the max torque point.

The only good way to determine this is to actually measure fuel flow (or air mass flow, which is proportional) at various speeds.

Isn't 0 miles traveled still MPG?

Yup, I was trying to provide a practical answer to the OP's question.