There is a small decrease in gas milage with the 3.73s, but its not that bad, you'll mostly see it if you do a lot of stop and go driving. The fun factor of driving goes up immensly though, and will improve your 1/4 mile too...(The modulars love the higher RPMs) ;-)
Be sure to get an aftermarket shifter such as a Pro 5.0 or Steeda Tri-Ax to go with your new gears. You won't believe you drove with the stock shifter afterward...
As far as the dynomax cat back, The sound is nice and throaty, but not loud and annoying as flowmasters can sometimes be. Don't expect any horses out of a cat-back only install though. If you can get them at a good price, the sound alone is worth it.
(In my opinion.)
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98 GT
Dyno-Max cat-back exhaust, 3.73 gears, Steeda shifter, Ford Motorsport Underdrive pullies, 87mm MAF and Roush cold air kit...
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