You'd be better off staying at the speed limit. The faster you go the more rpms, O/D or not, and more rpms means more fuel used to accomplish this.
Also unless the road is nearly flat O/D will be kicking in and out, when it kicks out, rpms will have to rise to hold the speed. Holding a steady throttle vs constant up/down uses less gas.
Plus staying at the speed limit will tend to keep your wallet from loosing weight when you get pulled over. LOL!
97 Sahara, 74 Renegade, 72 CJ5, 55 Willy's (the Beast) Boss 302, 6:34 gearing, Bose 901 speakers.
God I miss that Jeep. Forgot the kinda Jeep, 80 SX4.