Another item you might check is the fan clutch. If it is seized, then it will pull more air thru the radiator fins, than if it was free wheeling cold,then hooking up when the viscous damper got warm. Your Bronco is carb, not fuel injected, is that true? Mileage won't be affected if it is carbed, cause the choke is electrically heated, and will open up. If it is injected, yes mileage will suffer because it won't get to operating temp, and the coolant temp sensor reads it is still cold and keeps the injector pulse rich. Kinda like a choke. Won't go into "closed loop" Put a stat in it, and check the fan clutch operation, if it has one. Or, put a chunk of cardboard in front of the rad.