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I recently purchased a prestine 1999 Mustang Cobra with 59k miles on it. I upgraded the exhaust with Borla 2.5 pipes. I am thinking about having 3:73 gears installed but I am concerned that the possible modification could not be worth the gain. Please let me know if I should upgrade the gears or leave them alone.

Thank you,

Mark
 

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Welcome. It really depends on what gear you've got now, and what you want to do with the car. If you're planning to race it at the drag strip regularly, you might want to consider 4.10's. My 08 GT came with 3.31's and while it got me where I wanted to go pretty quickly, I wanted to feel the acceleration more, so I upgraded to 3.73's. A big difference, its a lot more fun to drive now, and I am considering taking to the track to see what it'll do in the 1/4. If you've got 3.55's now, you may not see much difference going up one size.
 

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welcome to MM. you'll really like the 3.73's. a lot of people go 3.73 instead of 4.10 because they're concerned about the gas mileage, but from what i've heard from guys using 4.10's it's not that big a difference. either way, i would definitely upgrade the gears. it's a great mod...
 

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I gained about 3 mpg when I installed the K&N CAI (cold air intake), then lost about 2 mpg when I went to 3.73's. So if I can keep my foot off the floorboard it comes out about even. If you don't have a CAI yet, I'd recommend one. You can get the K&N for around $250 and there are others also, but some give so much more air to the engine that you need to reflash the computer to get the full benefit. You don't have to retune with the K&N, although it will help make a little more power. It depends on your budget, a CAI + tuner can run $600. The engine will breath better and produce more power, and it'll compensate your gas mileage, at least partially, for the step up in gears. All this is from my experience with the 3v 4.6L, I think you've got either a 2v or 4v, I'm not sure which, so someone with the same engine may give ou a little better info on the CAI.

Going from 3.27 to 3.73, you'll see about a 12.5% increase in RPM's at the same speed or a 12.5% reduction in speed at the same RPM's. So, if you were running 75 mph at 2000 rpm before, you'd be running about 65 mph at 2000 rpm after going to 3.73. So you'll need to adjust your speedo for the new gears also.

Also, check around your area for speed shops that can install the gears for you. Chances are they'll charge about 1/2 what Ford will want to do it.
 

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