[aerohead]

The beauty of the Kamm roof is that you can chop it (truncate ) anywhere you like.Up to a point,the longer it is the better, as it is allowing a deacceleration zone for the air to regain pressure so when it does break away from the car,the wake is at a higher base pressure with less drag,however as others will say "we don't live at Bonneville",so as reality intrudes,we're faced with making some practical considerations.For the Vibe,you can end the roof wherever you like,and a vertical chop is just fine,as this was Kamm's intent.Bare in mind though, that the shorter the roof,the smaller the benefit.Since the Vibe is a "wagon-back" its sides are also converging in with the extension,which allows the top and side air streams to meet at closer the same velocity so as not to crash into one another,forming power-robbing eddies.