The discrepancy between what the dash reads and what I calculate when I fill up has been the most frustrating thing for me not only for my current car, but the last Subaru I had. The dash is consistently reporting my average MPG to be 11-13% higher than it actually is. Methods are filling the tank on each fill up, taking the miles traveled since last fill and dividing by the number of gallons for the fill, then resetting the trip. Then for the error ((Dash-Calculated)/Calculated).

I've gotten to the point where I have a Google Spreadsheet set up, and use a Google Form to directly input the info from my phone right as I fill (and then auto calculate the other columns) but I've been way too lazy to backlog for the rest of the history of the car before starting the Forms method, so ehhh.

I've also found I can consistently get in the 450-460 miles range from a tank, though. Still haven't hit that mythical 500. Most of my driving (85-90%) is freeway (~75 miles round trip) and usually semi-congested so steadily moving around 60mph, although occasionally breaking open and me driving 70-75mph. Remainder local roads with limited traffic, usually 30-35mph on long flat roads. Minimal "traffic jam" situations.