Huh.

My 260 and Ford-O-Matic had been delivering about 14.5 MPG for routine city driving (18 - 19 MPG highway).

Then for the past five fills the city driving MPG went up to 15.5.

Today I got 16.8 MPG! Wow!

My theories:
Warmer weather is causing the automatic choke to disengage quicker.
The engine not being cold causes the gasoline to vaporize and burn better.
The "summer blend" of gasoline works better in my engine.