When I was attending GMI (General Motors Institute) in the 70's, I wrote a paper about all of the gadgets that claimed to increase you mileage by 10-20%.... there were multi-electrode spark plugs, carb base plates with little fans in them, special air filters, fuel additives, special fuel filters, oil additives,..... etc, etc. I added up the combined value of each product and came up with an engine that could produce copious amounts of gasoline..... :
On a serious note, putting in taller gears in the differential could very well have the opposite effect unless you only drive on flat-lands and never when the wind blows against you. Also, it would be quite cost prohibitive and even more than that, there are very few technicians that can set up ring & pinions properly and your chances of an early failure would actually increase. Having said that, you can install a GearVendors (or similar) overdrive unit on the back of your transmission. That would give you the advantage of running your current gear ratio while on hills or bucking a head wind or allowing an over-drive to kick-in while on the flats and/or tail-wind. This would definitely increase your MPG.