Hi,
I do not agree with you. My method is:
1) Fill tank to full
2) Reset the odometer
3) Whenever you like, fill tank again to full
The amount of gas you fill in the second time plus odometer gives you enough information to have quite accurate MPG. No need to know full capacity of tank or how much is left... Also, please note that the longer you drive in between, the more average the result is.
Andrew

 

The manual say 2 gallons. I filled it up as soon as the light came on, and it took 17.2 gallons for a 19 gallon tank. I wanted to know also.