Safe depends on how much of the system you removed. As long as the pipe terminates after the heat shield on the rear, you won't be spraying hot exhaust gases on your gas tank. If it terminates before then, you COULD potentially be increasing the temperature of the gas tank.Of course, what you did is horribly illegal is most locales. Mufflers are required by law. They don't have to be the original muffler, they just have to be there.

Mileage as of last fillup: 24479
Economy (Lifetime/Worst/Best): 29.92/26.67/34.72 MPG