after all the kidding aside the biggest enemy of highway gas milage is the BTU's of energy required to push the car through the atmosphere at speed. Example: From an old Scientific American Magazine --Half of the energy required to make a bicycle go 30 mph is used to simply to overcome wind resistance.
70 mph takes a certain amount of hosses, BTU's, or Gasoline due to frontal area of vehicle. Each type vehicle is different, what engine speeds/gearing you select to produce that horsepower is a way less important factor-just so you are using the carb's main metering system only and staying out of the power stage and the idle circuits. The absolutely cheapest change (and most effective) you can make is to drive a little slower.
As far as "excessive rpm" wearing out the engine, I've personally seen a '54 chevy 235 have only a half thousanth wear on its rod journals after some 83,000 miles of running highways at 60-70 mph WITHOUT an oil filter ('cause they didn't come with 'em) but did have regular change intervals, non detergent oil. The mains had less wear than that. This was a stovebolt with insert type bearings and the factory full pressure lubrication system, 26" dia.(6.70 x 15) tires and factory stick shift with 3.70 gears (thats 2800 to 3200 rpm range). Dont be afraid of a little rpm, it wont hurt ya. It will help keep the condensation outta the oil pan.
