Ford Mustang Forum banner

1 - 9 of 9 Posts

·
Registered
Joined
·
520 Posts
Discussion Starter #1 (Edited)
Hi, i want a little more oomph to my 1966 mustang and i realized gears are the way to do it. I would like to change to a limited slip differential and i would like to know what all should be accounted for during the swap. Will 3.73 gears hurt mpg even considering i dont drive it even 50mph often, as it only brings me to school and work everyday. Most driving is 35 to 40 mph so i doubt it would do much difference, i hope. And for the swap, can you just buy the limited slip unit and install it with new gears? I think its as simple as that but i like to be sure before i dump a couple hundred on a part. I was thinking of buying a so called "8 inch Posi" unit from jegs or so for round 350 to 400 dollars. Will this just do the job? Im a virgin to these rear ends:hihi: so someone please enlighten me. Thanks in advance.
 

·
Banned
Joined
·
9,214 Posts
373s would be great. i dont know about those engines, if they are anything liek classic ponitacs and love low RPM's, id stay where your at. if not...373 :D
 

·
Registered
Joined
·
386 Posts
3.73 gears are pretty low for the old mustangs unless you have put in a 5 speed or an automatic with overdrive. Most people would not want to go over 3.5 gears but if you don't plan on running it 65 or 70 on the highway than the 3.73 will probably be alright. I don't know how much it will affect your mpg. Changing the whole center section or third member is easy but installing a limited slip differential into one will be more work and you might need to take it somewhere. The best thing to do is get a complete center section with the limited slip and the gears you want but they can be hard to find and new ones are expensive.
 

·
Registered
Joined
·
1,491 Posts

·
Registered
Joined
·
527 Posts
They aren't very hard to change. 3.73 gears will do fine in it. Richmond gears has a calculator where you can put your information in and it will give you the rpm's you will be running at different speeds. Don't forget that you need a gauge to check the play in it for your shims. I am sure there is a book or video somewhere about swapping out rear ends. At the speed you are driving you won't notice much difference in mileage. At highway speeds, you will notice a lot.
 

·
Registered
Joined
·
1,791 Posts
Ron I love your site and you have done some great work! I have just now conquored the rear end monster that can drive you insane. I have a few mustangs and one thing I have never doen was go through a rear end, I guess i feared it as some mystical thing I could'nt do. It took me two tries to get my 3.70 9" quiet but I did it! I bought a 1400$ currie centersection and they told me it whined because my hosing was bad - that was crap! I got my money back and bought Motive gears - they are the quietest and the install kit. All you really need is a Dial indicator to set backlash and a 6" caliper style micrometer(harbor freight for both, you dont need Starrett tools for this) My advice is save your money from buying ebay junk(non returnable) and check out summitracing.com and get some gears and a posi unit and install them yourself. Be clean and dust free in your work space (not painting quality but not dirty and gritty) and give it heck man. Do like you did with the paint booth and educate yourself and prepare for a LONG weekend. These chunks are heavy and nasty and oily and will slice and dice you, wear gloves and keeps lots of paper towels handy. Keep in mind that the rear is quiet now, and everything inside is factory - take LOTS of picks on teardown, you will setup your new gearset in to VERY close to where the side adjusters are now and your pinion depth will be written on your new gears, that will put you in the ball park, but know right now you may have to go back in at least once.
Now about that gear, that is steep with a 3 spd auto, you may run hot and forget taking any kind of trip over a long distance. I reccomend either 325's or 3.5' at the most, you will run out of transmission before you run out of gear. How tall are your tires? currieengineering.com has a very helpful calculator to show you how your tire and gear choice affect final drive. is your trans in good shape? launch hard with 3.73' and you'll soon find out. You may also develop some driveline vibrations with that gear, keep in mind you will increase the rotation of everthing by over 30% if you have the usual 2.79 now. Just food for thought man, go 50 mph and pull the shifter into 2nd gear - that is where it will be with 3.73's - all the time - everyday - everywhere you go - like to the gas station - frequently- Think this through bro.
 

·
Registered
Joined
·
1,491 Posts
Good info silver69!
I have always been afraid to tackle a rear end because of warnings to NEVER attempt it from others. It sounds like I may have to look into doing it myself if I can change the gears and buy the tools and still figure it is worth my time.
I have a 5 speed in The 66 and right now it is lugging the engine at freeway speed with 3.00 gears and running 225 VR50 16 tires. Doing 60-65, it is in between 4th and 5th gears and I need to go to 4th on hills or to pass. It is also making whirrring sound when I let off the gas. I hope it will last for the summer.
I will check out Summit and their parts.
Again thanks. I am tempted to do it now after I find out more information!
Ron
Christophers 66 Mustang Restoration and Modification
 

·
Registered
Joined
·
539 Posts
Did he ever mention what tranny he has? I think 3.73 are going to be too low for that car if it's an automatic. I was tempted to go 3.80 with mine, but I'm glad I didn't. I have 3.55's in mine with the C4 and I'm taching 3 grand on the highway.
 

·
Registered
Joined
·
1,791 Posts
Hello Ron,
as I said I too was hesitant but I got tired of paying big bucks and getting crap. Read up, you can definately do this. I failed to read your bio to see that you have a 5 speed manual trans in which case 3.89's would be nice too.
have you thought about stepping up to a 9"?
9" parts are more readily available - a tad cheaper and you will have a greater gear selection. Good luck and let me know if I can help you,
Joe
 
1 - 9 of 9 Posts
Top