Should'nt kill your mileage May even help To install the module you remove your ECU pull the plug ,clean the contact point, install the module trying not to bend the contacts and then tune it as long as you know what your doing with the tuning
And don't believe the HP increase numbers from the manufactor
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Mods-MAc CAI , True Dual Flowmaster 40,3.73 ,T-Lok, 25% UDP, 3/8 Phenolic Intake Spacer ,Steeda Tri-ax, SCT Chip/ Tune