Why our Prius "wanders"

This explanation says it more completely than I could, although less
simply... It doesn't refer directly to mpg, but to power loss which
causes the reduced mpg.

Selected excerpts from: Pointed the Right Way
http://www.ozebiz.com.au/racetech/theory/align.html

story by john hagerman

UNDERSTANDING TOE

"When a pair of wheels is set so that their leading edges are pointed
slightly towards each other, the wheel pair is said to have toe-in.
If the leading edges point away from each other, the pair is said to
have toe-out. The amount of toe can be expressed ...as the difference
between the track widths as measured at the leading and trailing
edges of the tires or wheels. Toe settings affect three major areas
of performance: tire wear, straight-line stability and corner entry
handling characteristics.

"For minimum tire wear and power loss, the wheels on a given axle of a
car should point directly ahead when the car is running in a straight
line. Excessive toe-in or toe-out causes the tires to scrub, since
they are always turned relative to the direction of travel. ...
So if minimum tire wear and power loss are achieved with zero toe,
why have any toe angles at all? The answer is that toe settings have
a major impact on directional stability. ... With the steering wheel
centered, toe-in causes the wheels to tend to roll along paths that
intersect each other. Under this condition, the wheels are at odds
with each other, and no turn results.
...
"The toe setting on a particular car becomes a tradeoff between the
straight-line stability afforded by toe-in and the quick steering
response promoted by toe-out. Nobody wants their street car to
constantly wander over tar strips-the never-ending steering
corrections required would drive anyone batty. But racers are willing
to sacrifice a bit of stability on the straightaway for a sharper
turn-in to the corners. So street cars are generally set up with
toe-in, while race cars are often set up with toe-out.
...
"Remember also that toe will change slightly from a static situation
to a dynamic one. This is is most noticeable on a front-wheel-drive
car or independently-suspended rear-drive car. When driving torque is
applied to the wheels, they pull themselves forward and try to create
toe-in. This is another reason why many front-drivers are set up with
toe-out in the front."

And so it appears that if the toe setting is zero, we get max. power and therefore max. mpg. but that leads to wandering, which is why we wander or at least most of us. apparently by re-setting the toe, we will reduce wander and increase gas consumption.

I think.