Topping off the tank is bad why?

[PDenbigh]

Same here Chan. I can get an extra 1.5 gallons in the F150, which in reality only works out to ~4% but hey! That's still somewhere between 4 and 25 miles depending on how much turbo whistle I want to hear. I imagine it varies by vehicle. If I'm filling the charcoal canister, somehow it's getting drained and used. I'll count that as an auxiliary tank! Otherwise, I do mess up sometimes and it comes spewing out of the filler. Whoops. Stinks on my shoes for the rest of the day - that'll teach me.