dont take this all that seriously but how fast do you reckon an fi car could go backwards if the gears where changed so the ratios wouldnt hinder the speed. its very difficult to say anything, id maybe say a somewhere under a hundred kmh, as i dont think the wings and diffuser would work that well in the opposite direction
i hope max mosley doesnt read this, forcing the f1 cars to go backwards would certainly increase overtaking and reduce speeds