Rear gear mystery???

[Doubleclutch]

Hi Guys

I have a 73 RR GTX 440 that appears all original BUT?? it has a performance gear that leaves it buzzing out on the road. I'm thinking of going back to a 3.23. Tack shows 3250 at 60.

Its the 8.75 rear and its coded last 3 "489". The gear shop advises that the rear #489 can't take a 3.91 gear??? max is 3.55.
It this true? I'm not ready to spend the time and money to drop 3.55 to 3.23.

Speed is correct 60 is a mile a minute out on the road. Maybe tack is out?

 

[benno440]

of course it can, can take any gear. 489 is the best centre chunk for the 8 3/4 too

 

[Badvert65]

I once had a 73 Road Runner with a 3.91 sure grip in it. I think it was factory. I kept the gear when I sold the car and it is now in my 65 Fury. They aren't highway gears. I usually watched the tach and kept the RPM's to around 3000 on any long trips (longest one was about 500 miles).
Measure the diameter of your rear tire and I can do the math to get your rear gear ratio.

 

[Hemi Rebel]

Find another gear shop. These guys are stuck in 12 bolt land or something. If you are taching 3250 at 60 mph, depending on tire size, i would say 4.10 or 4.30 in it now. 3.23 would be a big improvement

 

[Cranky]

Yup, find another gear shop because they know nothing about Mopars!! For one, like was already mentioned, the 89 case will accept any gear ratio offered. Not sure anymore but I think it goes all the way to 5.13's. Also, your 8 3/4 housing will accept any drop out third member offered my Chrysler but I wouldn't mess with anything older than 1965. There's always someone here selling complete third members ready to plug in and go. 45 minute job (or less) using a lift and air tools and someone with experience.

 

[hashmaker]

I agree with everything said, you must have 4:10 or 4:30 gears, if you go to 3:23 it will be like going to the other end of the universe. I went from 4:10 to 3:55 and thought it was perfect. If your tires are 27 inches tall or more 3:23 could be too much.

 

[bm02tj]

Lift the car and get two buddys and mark the tires and drive shaft then have them slowly turn the tires one complete turn and you can count the turns on drive shaft now you know your ratio
then calculate what rpm you want for ratio to change to

 

[Jackpot50]

Your shop is a moron! put in what you want, but go to another shop.

 

[Hemi Rebel]

And as if heaven is watching...

http://www.forbbodiesonly.com/moparforum/showthread.php?92641-3-23-sure-grip-for-sale

 

[62 Dart Convertible]

489 is the best centre chunk for the 8 3/4 too

Hey benno, I'd question that.

The 489 may have a thickest pinion shaft of all but that is not the weakest link in the rear end. When did you last see a pinion shaft snap? The 489 crush sleeve wasn't the greatest invention either.

Most say the 742s power-lok clutch pack is a better set-up than the 489s cone clutches too.

As far as ratio, for a city car, I would not go numerically lower than 3.55...

 

[mopar 3 B]

Is this a manual or automatic transmission car?

 

[Doubleclutch]

Not the shop-- he called his gear source. Not sure which one.

Auto 727

I'm going to have the tack checked as well as the spin the wheel suggestion. Shop has a timing light that shows RPM.