I found this interesting little calculator that will provide how many watts you put out for your weight and average speed and the equivalent in mpg of gasoline. Sure this may or may not be very accurate, but it's fun.

so at 200 lb and 15 mph for my commute
power raw/real (watts):
209.35 | 872.29
Calories per minute, hour, and mile:
12.51 | 750.5 | 50.0
Approximate Miles per Gal:
619 MPG

give it a go just for kicks
[edit]
http://ijar.chiggins.com/bicycle-mpg-calculator/

I fail