This is a problem probably very few will have. I'm an American living permanently in Canada. I buy my gas in litres and pay for it in $/litre. My cars' odometers show in kilometres, except for a classic car I imported to Canada which has an odometer in miles. But I still think in miles/US gallon. 'Litres/100km' is meaningless to me.
Is there a way that I can work with my Fuelly app and online dashboard so that I continue to enter data in metric, but have it show me gas mileage in mpg?
Yes, go to Fuelly's home page and click settings. There you can chose what to display your units in, US MPG for you. Then click on your vehicle, and click edit vehicle. Here you can choose method of tracking (trip odometer or odometer total) units (miles or KM) and how to track your fuel usage (litres, gallons etc)