I bought a Diablosport Predator for $334 plus tax, and the improvement was literaly like driving a totally different car. All you do is plug it in to the port under your dash and follow the instructions (same as most tuners) and it will walk you through the instal. Took me 15 minutes and It was the best money I have spent yet. It probably only yielded me 15 hp gain, but I installed the "firm shifting" program, and it was like somebody had given me a rebuilt
trans with a shift kit. Has your stock v-6 mustang ever chirped the tires when shifting? after my program mine did WHOHO! I am not pluging Diablosport, because there a a bunch of different ones out there, but a tuner is surely the first thing to do imo.

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2005 Torch red, predator 93 octane, K&N filter, Pypes true duals, Eibach Pro-kit springs,4.10 posi 8.8 rear end. GT fanblades with Eagle F1's, GT500 rear bumper/spoiler/front splitter