1999 cobra gear

My advice is to drive someone else's "geared" cobra, if possible, and then decide.

Will 1st gear "smoke" your tires after a gear swap? Maybe. Depends how aggressively you pedal it. Will 1st gear seem "really short?" Possibly. That's the idea of gear/torque multiplication though (when you change out a "tall" ratio for a "shorter" ratio, like going from 3.27 to 4.10 or 4.30, etc).

Try reading this, here is a direct quote of what Lidio from Alternative Auto recommends (a nice summary, IMHO):

Taken from: 4.6L Mustang Tech Q & A

"...What rear differential gear do you recommend?...

...4V Cobra: The Cobra has an entirely different engine than most people are used to. It performs best at high rpms, upwards of 7000. It's power band is relatively high due to the deep breathing nature of the two intake valves. When gearing a Cobra, 3.73's just aren't enough. They don't allow the engine to reach its powerband quickly enough. 4.10 gears are an excellent choice and 4.30's are even better. This may sound very steep if you're used to the average 5.0 which needs to be shifted by 5600 rpm. But since this engine will easily rev to 7000 it needs the extra gear and it likes it. 4.30's are our #1 choice, naturally aspirated or supercharged. At 80 mph in fifth with 4.10's the engine only turns about 2700 rpm, completely tolerable...."

Click to expand...

As far as selecting a rear end gear manufacturer, pick whoever you want - there are many choices; Ford Racing, Richmond, Motive, etc, etc. However, I would be more concerned about who is doing the actual installation and whether or not they install it correctly. It's nice if your installer has handled an IRS gear swap before (or possibly risk paying more in labor charges). You want to make sure your installer gets the install right the first time, so you don't run into driveline vibrations, blacklash, chatter, and/or gear runout problems, etc.

Every now and then you will hear somebody complain about a particular rear end gear manufacturer but usually it's because their installer messed up the install.

FWIW, most folks will recommend Ford Racing gears...

YMMV

If you're not considering a blower or turbo -- GET THE 4:10's!!!

-Aaron

Go with the 4.10's. Everybody I know loves them. It pulls harder and feels great. You could also go with 3.73's but my suggestion is 4.10's