we had a z on the lot a few days ago(i work at a car dealership) and my uncle for his police department he uses his personal charger and challenger there the exact same car except the charger has WAY more room... and more can be done to it... i say go with the charger just because the z was nothing to brag about...... and the charger has more room and the statement about 13 is false highway if you keep it between 1000-6000 it will cut down to 4cyl and get you way above 20(my moms 5.7 hemi jeep does it) she gets about 22 highway in a car that weighs so much more...