ML270 CDI - w163 - how do you work out mpg?

[ml911]

Hello all - I just picked one of these up a week ago and trying to figure out how people work out mpg with such accuracy. Is there a setting/reading somewhere? The only way I can do it by dividing drop in fuel needle by miles travelled (manually) - is this the only way?

 

[wilsodg]

Brim the tank every time you fill it, and divide miles since last fill by the amount of fuel.. Or use fuelly.com.

Would have thought there is an on-board computer that displays consumption, but it's not as accurate as brim to brim