I would agree with Tpenfield,maybe a bit less than 3 mpg. A lot has to do with deadrise and how much weight your carrying. A SS prop that is correctly fitted to your boat would get you maybe a 1/10th to a 1/2 mile per gallon because of less slip and maximum efficiency gained from the prop compared to aluminum one just thrown on there from the factory. Engine size has nothing to do with your fuel mileage. We could put a 5.7 from the same year as your 3.0 on your current boat and get the same mpg going the same speed, its when your right hand gets heavy and the secondaries open up that results in more gas being used by the bigger block. Now you may have been trailering the 18' and buying gas on the street,if your slipping it expect to pay .50 to 1.00 more per gallon at the pump. If your traveling less than 20 miles per round trip, which most people in my area do,its not a big deal if you have budgeted it. You will enjoy the actual ride 100% more going from a 18 to a 26' , no where near the pounding you take with what you have now and the Mrs. will be just as thankful, maybe even help break the cabin in with you ,haha.
