i guess the cd depends a lot with the front bumper... somebody got a wind tunnel here ?
anyway, the 300ZX is 1,7 tons, so it's pretty heavy (but still, a friend of mine got one, twin-turbo, 450hp, and does 18-19mpg (okay that's not super great, but considering the engine, the output, it is ^^)
Will you try lightweight rims ? you can save a lot (if you remain calm on the pedal
) if you do a lot if city-driving - start n' stop.