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It ruins your mpg calculations for the tank you just completed and the tank you're about to run because the bottom of the fraction gets a lot larger relative to normal. For instance my average since mid July has been 48.8 MPG. The tank before I started my 814 mile run came in at 24.8 MPG and the fill up afterwards came in at 69.9 MPG (first click was 73.3 MPG). The fuel range is computed by taking a rolling average of some unknown miles (I'm now thinking 50 miles) times the amount of fuel left in the tank. The fuel level sensor floats on a rail and when it hits the top of the rail it can't register anymore. In the ECO MT the top of the rail is hit right around 12 gallons, so any fuel you put in above 12 gallons won't "increase" the range calculation because the sensor simply can't report it. What you will see is your range fluctuating based on the rolling average mpg as you drive until the fuel level drops to a point the sensor is floating again. My range fluctuated from 655 to 685 miles for almost 250 miles - remember I was running 52.5-53.5 mpg that entire time. This is also the reason the car can't report range for less than about 0.8 gallons - the sensor is "sitting" on the bottom of the rail.
I'm convinced that the evap systems can handle this as long as you don't make a habit of filling into the fill pipe and thus flowing gas back into the evap system via it's vent. The evap system's vent is near the top of the fill pipe. Damage to this system occurs when the charcoal filters get clogged with the additives and detergents that are added to the gasoline. These additives are needed to keep the fuel burn clean but they won't evaporate back out of the charcoal.
I'm convinced that the evap systems can handle this as long as you don't make a habit of filling into the fill pipe and thus flowing gas back into the evap system via it's vent. The evap system's vent is near the top of the fill pipe. Damage to this system occurs when the charcoal filters get clogged with the additives and detergents that are added to the gasoline. These additives are needed to keep the fuel burn clean but they won't evaporate back out of the charcoal.