What can I add/install onto the Prius to reduce highway air drag/vortex movement?

That's a bingo!

The longer wheelbase of the Fusion also helped with high speed handling.

 

Tires make a difference also, including something as simple as inflation pressures.

 

Your Prius is only a couple hundred lbs lighter, but the wheelbase on the Ford is 6 inches longer. Additionally, it's tires are much larger (30mm or 40mm larger width) and probably made more for general roadholding than efficiency. Fins aren't going to do anything to keep you from being tossed about by gusts, I would get bigger tires is probably the best suggestion. Or a good alignment. More toe in and more camber would help with dynamic stability, at the cost of efficiency.

 

Dynamic stability is complex and affected by a variety of factors, including aerodynamic stability (surface area, side force, yawing moment, yawing moment change with yaw angle), suspension and tire factors, and vehicle loading.

You can measure stability using steering angle as an indicator. I found that shifting a 300 lb load moved the center of gravity of my car 3 inches, and this change was measurable in the steering angle in a constant crosswind, logged on an OBD scanner. Read more here. More on aerodynamic stability in general can be found in Julian Edgar's Vehicle Aerodynamics: Testing, Modification & Development (2023).

 

Only question is, how does one go about finding a constant crosswind? And verifying that it is constant for the duration of your test?

I will be happy to read your link later though - ex-FSAE member here, now an advisor for the local solar car team.

 

I have side and rear side splitters, they work great stabilizing straight line driving.

 

Side skirt extensions?

Retro F1 tech: The ground effect era

 

Side skirt extensions?

Retro F1 tech: The ground effect era

 

Oh nice, I thought about joining FSAE but Solar Car seemed more up my alley.

I check weather forecasts and wait for a day with suitable predicted conditions, build everything I'm planning on testing so I can quickly install or remove it to shorten test time to a minimum, and use an anemometer to check wind speed. Depends where you are, too--here in the flat Midwest in the winter, it's relatively easy to find suitable days for crosswind testing (windy but not gusting); where I grew up in mountainous Washington probably not so much (although I've never tried it there). I've abandoned many a test when conditions changed. By its nature, testing in the real world will be subject to variable conditions, so when I do things like coastdowns I also run statistical confidence tests on the results.