[MetroMPG]

I think Tourigjm hit the nail on the head. You could theoretically design a front-engine car with the same Cd as a low drag rear engine setup, by using active cooling inlets/radiators on the side of the vehicle, and waste heat ejected into the low pressure wake. Of course the plumbing/ducting would be a nightmare for the front engine version.

Another advantage to rear engine placement is you can adopt a 100% smoth undertray since there is no hot exhaust pipe running down the center of the vehicle that needs to be left exposed.