lol...why would you lock out 5th/OD? You most certainly WILL get better fuel mileage (common sense there) by running at a lower hwy RPM and it will be somewhat quieter. Kickdown or passing gear will be more because you're going from 5th to 3rd rather than 4th to 3rd, however not all downshifts will get you to 3rd....you'll just drop to 4th from OD (5th) at light throttle. BUT...if you prefer it in 4th, leave it there and enjoy it!...it's your gas money.