I Need a Definition of 'Halo Car'

Does anyone have a good, quantifiable definition of halo car? I am writing a large document and need to clearly define the term 'halo car' in order to easily (mathematically) separate it out from any other car.

I need to be able to create an equation, and then the reader can take any car and determine from that equation if the car is a halo model or not.

My simple definition of a 'halo car', a car that signifies the pinnacle of a firm's technologies, is not enough to mathematically separate out a car from the field. I may have to separate out and only focus on performance halo cars because they provide quantifiable figures (horsepower, torque, lap times, etc.)

Lap times won't work for the lack of sample data.

I've thought of assuming a performance halo car to exist under the condition: x% greater horsepower than the average model's horsepower.

However, what does this percentage of performance definition do for cars like the Tesla Roadster? Do I add weight into the equation; can I add weight into the equation?

And then there is the question of 'green halo' cars, MPG? Yet, it can't just be MPG because, for example, the Prius is the green halo car for Toyota, while the Camry Hybrid is just a covert hybrid model. Perhaps, MPG plus hybrid/electric initiated model (the model is introduced as a hybrid/all-electric, so as not to confuse).

Any ideas?