Fuel Economy..?

I have been driving the wife's 73 Dart, 4 door, 225 slant, with stock size radial tires, daily for the last month. My daily drive (round trip) is 74 miles, 95% highway, and I don't go over 60 mph.

Mods: Fuel line mod, timing at 6 btdc, plugs, cap and rotor as recommended on slantsix.org

My average mpg, calculated by tracking miles driven divided by the gallons on my fuel receipts is 23.7.