I work for Mitsubishi in the Uk and we have the 1124cc 3Cylinder Mitsubishi engine in our Colt
Overall MPG combined average is 51.4mpg (uk gallon) running with 95ron in a heavier 5 seat car. than the 42
I can tell you ,this engine seems to need a little running in, time to loosen up. Your mileage should improve after a few thousand miles
My Smart Roadster has the previous Suprex 698cc Turbo intercooled unit .Tank to tank fills give me 48ish mpg (uk gallon) it does not seem to make any difference if I give it beans. or drive Miss Daisy