Your choices for a General Motors overdrive gear are .63, .68, .72, .73, .78 and .86, but that is ALL of the GM choices. Some of those are cars with the tail shaft shifter. If you already have the .63 or .68, I don't know how you could do any better and if you have the .72 or .73, the difference between .68 and .72 or .73 is going to be not that noticeable. If by chance you have the .78 or .86 overdrive, than you might be able to help your self by dropping to the .63. That is a big drop and I can't see how over drive (5th gear) can be much good in the hills. I would think it is more of a a flat land, straight roads gear.
