Currently, I have a 2.73 stock gear ratio in my Mustang V6.
However, I would like to mod it.
I've googled and it's told me that going over 3.31 is a waste.
How high can I go?
Is it expansive to mod the gear ratio?
Is it self-do-able?
Is the mod actually noticeable?
In addition, could someone give me an easy explanation about what it actually does? I have a little bit of info on it from googling, but I could use some more info.
Thanks!

 

3.73 or 4.10 is as high as is feasible to go.
It costs anywhere from $500-1000 in parts and labor.
Not unless you change gears for a living, so no you need to find a good shop.
Gears are the single best NA mod for the 3.7.
Basically you are using a steeping gear reduction set that transfers power to the wheels. There are trade offs where you go faster but lose MPGs. Ford tilts things toward higher fuel economy.

So to give you an idea what happens, my car went from a 5.2 second 0-60 to about 4.7 seconds with just 3.73 gears. I started with 3.31s from the factory. Your car probably takes about 14.2 seconds to go 1/4 mile. With 3.73 or 4.10 gears, you'll drop the time to about 13.2 seconds. You will also need a tune so ensure your speedometer is correct. You can't take it to the dealer because they aren't capable of doing it. I've got a bunch of other mods and can snag 60 mph in about 4.2-4.3 seconds now. I'm not sure about my 1/4 mile time but would guess it would be somewhere between 12.9-13.2 seconds.

 

Bro, no waste at all. 3:73 or 4:10's are the bomb. I have 3:73 gears in my 3.7 and can roll 0-60 mph in 4.7 seconds.