I am trying to figure out my rear end ratio. I have a 83 280zx. In 5th gear i can do about 65mph at about 2800 rpm. I think i have 3.9:1 Also i have seen people mention 3.7:1 and 4.11:1, which will give he more top end cruising speed? The previous owner had dunlop tires, which i need to replace. Should i stick with these or change ? Any suggestions? I am looking for quiet, good riding and above average handling. I am looking for something to cruise the highway with.