# Rear end ratio question



## jpdog357 (Feb 27, 2006)

Guys: 

I have decided to have a new limited slip carrier put in my 1972 400 Lemans and am considering swapping out the gears at the same time. 

I am looking for feedback on what ratios were stock on a GTO vs. my Lemans as I am not even sure what it has currently in the car. 

I am considering waking up the ol' girl soon with a 4 barrel and manifold but won't be going beyond that for now.

Also, any feedback on what is a good all around streetable ratio as I won't be racing the car and at best would like some street-light fun but also take the car on long highway drives of 100 miles or more on occasion. 

Thanks, 

Jason


----------



## SANDU002 (Oct 13, 2004)

Jason,

Go with 3:42's


----------



## BobG (Dec 20, 2006)

Is your tranny 3 spd manual, 3 spd auto or 4 spd manual.

3.42 wasn't available on a 72 goat.

available ratios / engine combos were:

Rear End Gear Ratios 
Engine Trans 3.07 3.08 3.23 3.31 3.55 
Std 250hp 3-Man . . Opt . Std 
3-Man w air . . Std . . 
4-Man . . Opt . Std 
4-Man w air . . Std . . 
4-Man Close . . . . Std 
3-Auto . Opt . . Std 
3-Auto w air . . Std . . 
455cu in 3-Auto . Opt . . Std 
3-Auto w air . Opt Std . . 
455-HO 4-Man Close . . . . Std 
4-Man Close w air . . . Std . 
3-Auto . . . . Std 
3-Auto w air Std . . . . 


This data is courtesy of ultimategto.com. The standard 250 hp motor is the 400 ci. Personally, if auto, I'd see if I could find a 3.23, even if it doesn't have a/c. that is, if you plan on the drives being more important than stoplight grand prix. If you want the giddyup to be more important, go with the 3.55. With the 3.55, you'll be turning just under 3 grand at 65 mph with the stock wheel/tire diameter of 26.8".

To look up the current ratio, there is a letter code on the tube, either right or left side. that code corresponds to a gear ratio. I'd give you the codes from my restoration manual, but it only goes to 70.

I have a 3.90 in mine, and it's great on takeoff, but I usually tend to drive back roads rather than interstates, because at 65, I'm turning close to 3300 rpm.

hope this helps


----------



## 68goatconv (Jun 12, 2007)

jpdog357 said:


> Guys:
> 
> I have decided to have a new limited slip carrier put in my 1972 400 Lemans and am considering swapping out the gears at the same time.
> 
> ...


I think that originally it was probably 3.08. I put 3.31 chevelle gears in my '68 and run almost 3000 RPM at 70 MPH on 15" tires. For trips, i'd go with something a little less (gas isn't getting any cheaper). The trade off, of course, is low end acceleration. In '68 the ratios were: 2.56, 2.78, 2.93, 3.08, 3.23, 3.36, 3.55, 3.90, & 4.33. The last two were only available in the 4 pinion locking differential. Don't know about '72.
Ed


----------



## jpdog357 (Feb 27, 2006)

Thanks for the prompt feedback.

Its a 3 speed auto with air. Sounds to me like the 3.23 is a good option. With getting a little more punch of the Lemans it sounds like a good fit.

Jason


----------



## lars (Sep 28, 2004)

You most likely currently have a 3.08:1 rear. Swapping to a 3.23 is only a 5% difference, and you'll never even notice the change. If you're wanting to noticably change the performance and feel of the car, you need to go 3.55:1 or steeper. The 400 runs fine at elevated speed with a 3.55.


----------



## jpdog357 (Feb 27, 2006)

Guys:

Just picked up the Lemans. Had an Auburn limited slip carrier and 3.42 gears put in. Turns out I had 2.73s in it stock.

So quite a difference. The 3.42s are certainly hotter off of launch but the revs on the highway seem tolerable. With the 3 speed automatic it doesn't take very long to get to high gear in normal street driving.

Now I need to try to recalibrate the speedo as its out of whack. Any recommendations are appreciated.

Thanks for all your feedback, I appreciate it.

Jason


----------

