Hot Rod, thanks for the great info. I have a 66' GMC 'Burb with 305 V6 and looking to make my daily driver. Been wondering how to check the rear gear ratio and your explanation was timely. I'm guessing the 3.75 ratio would be the best for highway speeds ( 60 - 70 mph )? If not, what would the best ratio be while keeping the RPMs low and MPG high? Also, how difficult would it be to change rear end gears. Oh, it has the factory 3 on the tree. Really a great truck. Thanks again.