Unstable steering

Unless you plan to get rid of the problem car soon, you might still find it worthwhile to pay the diagnostic fee (1 hour labor) and ask to have the steering sensors calibrated? Especially if you bought the cars from the same dealer and explain the fact you have been blaming the issue on the tires so far but now have learned about sensors in the steering that may need calibration, they might look at it. My guess is the whole thing may not cost more than a couple of hundred dollars. They will typically want to start with an alignment check and then might follow thru with calibration if you can show you want the steering to feel and behave like your good car as the reference.

 

I also happened to post my question to AA1car.com and Larry kindly responded:

Your car should NOT be wandering or pulling to one side, especially at slower speeds.
Your car has electric-assisted steering. There is a torque sensor in the steering column and a steering angle sensor that tell the steering control system which way to add power assist (left or right). I suspect something is misadjusted or defective in this control system (possibly the torque sensor, or the steering angle sensor needs to be reset).
If your Prius has a LOT of miles on it (over 100,000), your steering wander problem may be due to worn wheel bearings in one or both front wheels, or worn tie rod ends. These parts should be inspected and replaced as needed.
There is also a technical service bulletin your dealer shoudl refer to for correcting steering pull problems:

T-TCI-3964
February 27, 2013
REPAIR MANUAL SUPPLEMENT: VEHICLE PULLING TO ONE SIDE

Larry
AA1Car.com

 

I think Larry meant TSB-3964.