Is that what those things are on random civics? Do they make any difference for regular street cars?

I put some (actual, as in from an airplane parts catalog) vortex generators on my hybrid. It slightly increased gas mileage and slightly reduced noise.

The less aerodynamic the vehicle, the more noticeable the result will probably be.

>It slightly increased gas mileage and slightly reduced noise.

Between the day to day road conditions and different routes noise and the modest effect, I'd say this would be impossible to tell.