MPG is measuring fuel ecomony - how far you go per given amount.
L/100km is measuing fuel consumption - how much you use per given distance.
As they are using a different viewpoint the conversion is not overly simple.
One (UK) gallon is 4.55litres.
100km is about 62 miles.
So 9.1l/100km would be roughly 31mpg
To convert from mpg (UK) to l/100km: EG 35mpg
Divide by 4.55 to get miles per litre. EG 7.7
Multiply by 1.6 to get kilometres per litre. EG 12.3
Divide 100 by the km per litre figure EG 8.1
To covert from l/100km to mpg (UK): EG 9l/100km
Divide 100 by the l/100km figure: EG 11.1
Divide by 1.6: EG 6.9
Multiply by 4.55 to get mpg (UK): EG 31.6