[wagonman76]

Id try swapping some larger tires on just to see how it does. Thats a great idea as long as they fit.

And if it proves well, Id bolt on a junkyard rear axle assembly rather than try to change the gears themselves. Ive read a lot about changing gears in the RV towing forum and usually it requires setting up precise clearances with shims, if its not done right youll burn up the gears in short order. It takes a pro to do it right which costs quite a bit. Then too if you ever want to go back, just bolt the rear axle back on. You might also have to reflash the computer, or get a VSS signal adjuster box, Im not totally sure.

My take on it is to try to get the most distance out of your available torque, without having to downshift. I would think a Mustang would have gobs of leftover torque that is reserved for performance driving. Change the rear end and you probably wont be able to burn the tires, but youll get farther down the road instead. Now if you change the gearing so far that you end up having to downshift on mild grades and headwinds, then its not worth it.