I am new to this forum. I have an all original 66 GTO, 389 with four barrel, four speed and 3.55 rear. I bought the car to drive and enjoy, at 10.5 miles to the gallon, gas prices are keeping me from driving very much. What is the best rear end for mpg and performance? I have thought about a 3.08 or 3.23, what can I change to with the least expense?