OHHHH! We arn't talking 1972 GTO, are we? ;-D
No, we’re not talking about a carbureted brick from GM with the frontal area of a billboard. cD on the stock XJ6 is about .46; however it has minimal frontal area, and I’ve installed a TWR spoiler that actually drops it to .39 (somewhat counterintuitive, but it actually works).
There are other factors that influence a vehicle’s fuel economy, other than simple air resistance. Engine powerband, drivetrain gearing, and fuel mapping all play a part.