Cool - learn something new every day on here. So the tank is almost 24 gallons. I could see getting pretty darn close to 420 on an empty tank (17.5mpg), but I'd be watching that "E" light and hoping for a gas station!
I get closer to 18.7mpg, on a normal work week (60 miles per day), but I never get much more than 340 to a tank of 93 Octane. I need to fill up after five days of going to work. Maybe my gauge needs to be calibrated or looked at or something. Wonder if it is reading lower than it really is.