It was listed as an option for the 2008 charger under the build your car. Gas mileage would not drop to single digits. The car has two over drives. In fact, in the city, gas mileage may not drop at all. It would drop some above 60 mph, but I doubt it would drop that much under normal driving. The main reason gas mileage would drop is because none of us could keep our foot out of it.
3.90 would be way too steep for one of these cars IMO, even with 2.82's it comes pretty close to the 1st gear ratio of a 727 w/3.55's, I'd love to have a set in the 3.2-3.3 range but theres no aftermarket support yet.