Out for a spin.

[Jeff W]

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.

My Theory: your co-worker is adding a gallon to the tank each day you drive to work. Wait until he starts the siphon in a few months...