Highway RPMS ?

I have found that running above 2800 and definitely over 3000 rpm, the engine seems to use more gas than it returns in extra speed. The car is a 73 Dart MoDo 205/904/2.73. The tires are 24.69" high (225/60-15)The car has a 2¼" exhaust system. I used to get 27mpg while averaging 65 mph. The only other mod done on the car was removing carb heat and dismantling EGR.

As far as prepping any car for a long trip; evaluate the general condition of the car. What's broken and needs to be fixed. Plan route to avoid hilly terrain,if possible, or at least plan route with gentlest gradient changes. Avoid major metropolitan areas during rush hour. Make sure you have plenty of cigarette outlets for recharging the electrical menagerie,

Depending on outside temps, I suggest a couple of gallons of water for a vapor lock. In a similar vein, suggest upper and lower radiator cooling hoses and the tools to install them if need be.