Po2800 car is wasting gas

Is this supposed to be an OBD-II trouble code? P2800 is a code for the transmission range sensor, or PRNDL selector being out of range, if that's what you're talking about.

What do you mean by wasting gas? Your gas mileage has dropped significantly; is that what you mean? If you're actually getting a P2800 code, it could be that the transmission has been set to a low gear so when you're at highway speeds you're revving the engine like mad, but that should be pretty noticeable.

You'll need to clarify your question if you want a real answer.