I didn't understand your question -- what exactly do you want? I don't play NFS so I don't know how accurately you can set the ratios, but the basics are the same for every racing game or sim: make sure the you almost reach the red line in top gear in the end of the longest straight of the track, make sure that the ratios of first and second gears fit the slowest corners of the track, and then adjust the ratios of all the other gears so you don't have "holes" between any gears.
If NFS only lets you set all the ratios as one, just select the best compromise between quick acceleration (low ratios) and high top speed (high ratios).