[aerohead]

I know of no general rules or relationships for G.C. and aero.For street driven passenger cars and light duty pickups,auto manufacturers design in "approach,departure,and ramp breakover angles,as per Society of Automotive Engineers SAE J689 Standard.This allows a 16-degree angle at the front of the car,from the tire tread bottom,projecting up and forward,clearing just under the nose.Behind the front tire,in front of the rear tires,and also behind the rear tires,they like to see a minimum of 10-degrees projecting up to the rocker panels.Anything less,and you leave parts of the car on driveway ramps and such.High performance cars and high mpg concept cars use active suspension,which lowers the car on flat open highways,and then raises the car back up when it decelerates for regular in-town driving.4-4.5-inches sounds kinda low.You probably have an advantage right now,aerodynamically speaking.Your challenge is to keep the car in one piece.