It depends on the combination. Do a search here on the Metro XFi and the regular Metro and look at the differences. If the combination is tuned right to take into account the gear change, the mileage will get better. Since the Mustang at some point with the 4.6 had something like 3.08 to 3.27 gears from the factory, it shouldn't be too much of a stretch.
It's all a compromise.
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Volvo WIA42 VED-12 / 335 hp / 1300 ft/lbs / 9 mpg
Big n' Boxy, Never met a Hill it Didn't Like
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