[lunarhighway]

maybe it has to do with where the engine air pickup is. on some cars the engine picks up it's air in front of the radiator, in some older car from inside the engine bay, but in both cases blocking the grill will cause warmer air to be ingested by the engine, and also a decrease in pressure might cause the engine to have to work harder to breath enough air.

in some cases warm air is a good thing for economy but it also leads to a decrease in performance wich could work out negative for FE

since the grillblock seems to work for aero you might considder rerouting your air pickup point to an area where there's higher pressure (the front of the car,) or maybe alter the grillblock so a little opening is situated in fort on the air intake so it car breath unrestricted.