It has no effect on MPG, it just let's you hear the whistle of the turbine as it spools up. If anything, you'll rev the engine higher and actually waste more fuel, but the sound is definitely worth it. Real, big-rig, whistle.
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520 Rear Wheel H. P. and 800+ ft/lbs Torque = 15. 5 sec @ 97 mph with the rear wheels spinning in 5th gear going through the traps in the quarter mile. No slicks here, pal, no fancy auto transmission, just the truck in full street trim, 7400 lbs. of Auto Wurks Diesel/BD Performance raging, fire-breathing DIESEL POWER