I have decided to go with a 3.55 gear for my 4 sp 66 mustang fastback. how difficult is that to install? can i do it? or should i take it to a shop? thanks! i have the tranny and drive shaft off already. thanks!

 

Without the knowledge and special tools, you won't be able to change it yourself. Take it to a shop.

 

Soaring nailed it! Incorrect pinion depth and backlash will make your expensive parts junk. If you're trying to save some bucks, remove the axle retainer plates, pull out the axles about 4", unbolt the third member from the housing and take it to the shop. You should be able to get your labor charge cut in half that way. Another option is to pick up a third member already set up with the gears and differential you want.

 

Not to be a wise guy, but if you have to ask you can't do it.

The job requires training and practice and special tools.

Follow the previous good advice.
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