I have a 1995 Ford F 350 power stroke manual 4 speed with overdrive. I bought this truck 7 years ago it had 135k miles on it now has 160k miles. For the next 6 years I plan on using it to travel the west ( home base Portland OR)visiting all the national parks with my son; camper on board and fishing boat in tow. The truck currently gets about 10-12 miles a gallon full, empty, uphill, downhill. I travel at 65 mph at about 2850 rpm my question is how to increase gas mileage around the 20 mi range, gear splitter, or new tanny and front differential best way to get there. I am installing a better exhaust system now. The tires are 35/8.