Dear Kevin,
Here is the formula :
X l/100KM => (X/3.785/100) G/KM 1 Gallon = 3.785 liters
(X/3.785/100) G/KM => (X/3.785/100*1.609) GPM 1 Mile = 1.609 km
= X*0.004521 GPM
X*0.004521 GPM => 1/(X*0.004521) MPG
= 235.2/X MPG
So, if you OBC says 21.3, 235.2/21.3 = 11.04 MPG
Damien Veyret
'94 318is