REAR GEARING ADVICE

[Weldguy]

While Ohio enjoys a "Tax Free" week, I've decided to order the new gears for the Challenger project.

I am undecided between a 3.55 and 3.76(?) ratio. It's a mildly built 440 with a new 2200-2400 stall converter, new TCI Super Streetfighter Torqueflite and 8 3/4 rear. She was a 12.90 second weekend fun machine, now returning to mainly street cruising. Probably very little highway driving and I would like to "light 'em up" occasionally (I'm 76 years old LOL).

Also, I would like to choose a good gear supplier. I've read good and bad about Richmond. Is there a more reliable gear source given I'm putting her back as a fun street machine.

Thanks again for your always sound advice.

 

[pschlosser]

I recommend the 3.55s.

4.10s can feel too low (a ratio) on the freeway when the RPMs are pegging 3500 just to do 55 MPH, and 3.23 (the defacto standard issue) with a 440 cannot keep the tires hooked. But the 3.55 is a nice compromise between the two.

I know a few cases, where people have swapped out there 727 for a newer 518 tranny that has overdrive. Now, that is a nice setup, if you want both low gearing for racing (be it 4.10 or another), but higher gearing for the highway drive home.

 

[Bret Schneider]

"While Ohio enjoys a "Tax Free" week, I've decided to order the new gears for the Challenger project."

What??? Our Iowa tax free weekend means we can just buy school supply crap tax free. And even that stuff has to meet certain requirements. Tax free car parts? I'm moving to Ohio. Just a couple letters different from Iowa anyway. Plus you have a football team with an offense.

 

[Weldguy]

"While Ohio enjoys a "Tax Free" week, I've decided to order the new gears for the Challenger project."

What??? Our Iowa tax free weekend means we can just buy school supply crap tax free. And even that stuff has to meet certain requirements. Tax free car parts? I'm moving to Ohio. Just a couple letters different from Iowa anyway. Plus you have a football team with an offense.

It used to be "back to school" items in the past and only one or two days with a $75 per item limit. This year they opened it up for a week and raised the per item to $500! There are some restrictions like tobacco, drugs and alcohol. We can even go out to dinner and not pay tax!!!

 

[Weldguy]

Another question... While checking Jegs pages, some listings specify, "Late 8 3/4."
My Challenger is a 1970. How can I tell if it's early or late??? It is the 489 case.

 

[Bret Schneider]

Lot of 8-3/4 info here
Chrysler 8-3/4" Axle Guide

"1-7/8" tapered stem pinion (aka. '489')
Carrier casting numbers: 2881488, 2881489 (1969-1974).
This assembly was introduced in 1969 and was phased-in to replace the 1-3/4" unit through 1970. Note: the 1-3/4" pinion also appeared in some '489' carriers during this period. By 1973, the '489' was the only unit available in passenger car applications. This assembly was typically used in high weight/medium horsepower applications through high weight/high horsepower applications.
Pinion depth is set with shims, preload is set with a crush sleeve"

Sounds like the difference might be found in the pinion size?

 

[Weldguy]

Lot of 8-3/4 info here
Chrysler 8-3/4" Axle Guide

"1-7/8" tapered stem pinion (aka. '489')
Carrier casting numbers: 2881488, 2881489 (1969-1974).
This assembly was introduced in 1969 and was phased-in to replace the 1-3/4" unit through 1970. Note: the 1-3/4" pinion also appeared in some '489' carriers during this period. By 1973, the '489' was the only unit available in passenger car applications. This assembly was typically used in high weight/medium horsepower applications through high weight/high horsepower applications.
Pinion depth is set with shims, preload is set with a crush sleeve"

Sounds like the difference might be found in the pinion size?

Sounds like pinion size. Thank you.

 

[MoparCarGuy]

Handy Excel spreadsheet table showing RPM at 70 mph for the five major rear gear ratios with a 275/60R15 (28") rear tire.
You can use the Tremec website's calculator as well.
The 727 transmission gear ratios are below for 1st, 2nd, and 3rd gear.

 

[Xcudame]

I, too, would go with the 3.55 gears. The 440 will be able to spin your tires easily and you won't be howling on the freeway when and if you have to go on the freeway.

 

[Weldguy]

OK. 3.55 it will be!

 

[Chryco Psycho]

I would also go 3.55 & replace the crush sleeve with a spacer

 

[Weldguy]

I would also go 3.55 & replace the crush sleeve with a spacer

Yep. Already have the shimmed spacer kit.

 

[Spyder]

Sounds pretty similar to my car, although I'm currently thinking about putting a higher stall TC in (I don't remember what mine is, and it's tight. Probably stock)

I bought a Dr. Diff 3.55 third member last fall and put it in. Love it. Kept the 2.76 single leg for if I ever plan a cross country drive with it, it lives in a bucket under one of the shop benches.