I can only imagine my V10, 7400# F350 without OD on the open road. At 71mph, the tach is at 1900-2100 rpm. Kick it out and into direct, and it's running 27-2900 rpm. It takes more fuel to maintain that level of engine speed and ground speed. Will it pull as much of a load in OD? No, which is why that particular trans has a load/haul switch.