Highway RPMS ?

OK, your only consistent grades will be the hills through PA on I80 if you head that way.

BTW...the Dart Lite that I drove for years had 2.76 rear gears and a 4 speed manual trannie, where 4th was a 0.83 OD. 65 mph was a solid 29 mpg. 70 mph it would be around 28 mpg out the flat land of the midwest. But, for steep grades I could drop to 3rd (1:1 gear) and it would have plenty of torque to do 80 mph or better up a 12% grade near my home in VA; it sure did not mind the revs in that mode.

So you are kinda moving towards a small OD gear with the taller tires.

(Of course the lighter weight of the Lite helped, and the carb was tuned for mileage from the factory.)

BTW, C130 Chief gives a good reference to the increase in wind resistance with speed; actully the increase is typically between the square and the cube of the speed so its effect is understated above. By 70-75 mph, wind resistance starts to dominate the added power needed to move the car any faster; it starts becoming a significant factor as low as 45-50 mph. Hence, your sweet post for mileage is typically around 50-55 mph, +/-.