I have an app on the phone where I put the ODO and litters of fuel used at each fill-up and gives me the exact MPG for the last fill-up and historical data. So far is the most reliable way of truly knowing how much fuel I'm using.
Im my old country we use l/100km which made more sense to me but after may years of adjusting to MPG and gauging fuel efficiency after it, I sort of miss it on the CR, especially with so many features on the trip computer.