To keep my Total Average MPG accurate for each tank, I reset it every time I fill up. You have to go through the infotainment system: Vehicle > Trip Information > History > Update (and make sure you select "Update" and not "Reset")
The Crown Platinum usually shows trip and average MPG not a direct tank MPG. But you can get a good estimate by resetting your trip meter every time you fill up and then doing a little math.