As long as your stereo AMP is not pulling more than the max. recommended amps for all electrical accessories ( And I don't know what that is ) I do not think the added stereo will void anything.

Want more MPG without much effort?
Make sure you have at least 50psi in your tires.
Want even more MPG?
Drive 5 miles per hour under the speed limit in town, and 10 MPH under the speed limit on the highway.

75 MPG @ 55 miles per hour is a reasonable expectation.

This drops from 75 MPG to about 50 MPG if you drive 75.