I think you'll be right around 20. The car is pretty light, but not very aerodynamic. The Turbo 400 is going to burn a little extra gas. In a car that light, a th350 might be able to survive and will use a little less fuel, but who wants to take the risk? Depending on the tire height, the overdrive may not be too necessary with the low rear gear. $1700 for a GV OD is a big investment on something that might only save you a few hundred a year and that is if you drive the car a lot.