So I recently bought a 2015 Murano AWD Platinum. How accurate is the trip mileage reader on the car? I haven't used a full tank of gas yet but it's saying around 19mpg in the last past couple of trips. What is the average between highway and city driving together usually? I'm was thinking based on the fuel specs I was going to get around 24mpg combined. I"m using 87 octane fuel.