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