/6 with 4 Speed OD, rear gear recommendations?

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Backtobasics

Backtobasics

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I have /6 Dart Lite.
It came from factory with 3 speed and 2.94 in 7.25 rear axle.

I have 8.75 that I want to swap over to. I also have 4 speed OD that is also going in at some point.

Question:
What rear gear would you recommend with the OD?
I drive this consistantly, 2-5 times a week, and most of my driving is highway (25MPH to 65MPH). There is some around town, but it is not huge.

I have a 3.23 and 3.55 at my disposal. Would 2.94 or 2.76 be too far fetched?

I do NOT need monster acceleration in this vehicle. It is staying /6 eventually getting a Grand National turbo with Offy 4 bbl intake, and 4 bbl carb. I like to play to see what MPG I can get.

Opinions?
 
YOu could probably get a 3:23 in the 8 3/4, very common, still get decent gas mileage and better acceleration. 2:76 might be an option if you want even better mileage at cruise speed.
 
I have a similar setup in my /6, and I'm planning to switch to a 3.23 ratio. Like you, I'm not a tire-screecher, and am looking to retain the awesome MPG I'm getting with the 4-speed OD and stock rear gears(2.76?). Most people like the 3.55 and up, but they might be more concerned about launch speed.
 
ElFin said:
I have a similar setup in my /6, and I'm planning to switch to a 3.23 ratio. Like you, I'm not a tire-screecher, and am looking to retain the awesome MPG I'm getting with the 4-speed OD and stock rear gears(2.76?). Most people like the 3.55 and up, but they might be more concerned about launch speed.
Click to expand...

Awesome MPG is what I am looking for. This car is for cruising and enjoying on a regular basis. If I want speed, I have access to a car for that. Funny thing is, the "fast" car has been out 3x this year, and I have driven the Dart 3x in a week.
 
I had this combo for a while with a 2.76 gear in a 7.25 rear. It was hilarious but I thought it was cool 'cuz I like doing 80+ on the fwy anyway. The take-off wasn't too bad.......just expect to take it outta 4th every time you approach anything that even LOOKS like an incline.
 
I put an A833OD in my 68 Barracuda when it had the \6. The car had 3.23 gears in the 7.25. A couple of years latter I built an 8.25 with 3.55 gears for the car.

There was absolutely no difference in mpg. Before and after the rear swap the car got 19-20 mpg around town and 22-23 mpg on the hiway. BUT, there was a noticeable seat of the pants improvement in acceleration with the 3.55 gears.

With 27" tall tire I am turning 2200 rpm at 70 mph on the hiway.
 
My Feather Duster is nearly the same setup (mine has the aluminum 833OD, I didn't realize they could come with a three speed). After I had the flywheel turned and replaced the clutch at 288000 a year ago last summer, I checked my mileage. 31.4 running 14 inch wheels with 205/75 tires. Not the 36mpg the factory rated them, but not bad considering the motor has never been touched. Around town mileage seemed to hang around 20 or so. A short time later I put on my 15 X 7 cop car wheels and 235/60/15 TA radials (same diameter as the previous 14s, but considerably wider). I haven't checked highway mileage since the change, but the wider/heavier wheels/tires seem to have cost me a couple of miles per gallon around town.

The 100 pound heavier 8 3/4 would probably cost mileage the same way. While 3.23s or 3.55s might not cost much, if anything, in town mileage, the highway mpg will probably drop considerably. 2.45s or 2.76s most likely wouldn't do anything for you but lug the motor down and burn out clutches.

Installing a T-5 (weighs about the same the all aluminum 833) behind the Feather/Lite slant with a similar OD ratio and a 3.50 or 3.35 first gear would give about the same acceleration of the other gear sets and not give up any mileage up top. The extra gear of the five speed might even improve the around town mileage a wee bit.

An 8 1/4 rear end would be plenty strong for the slant, is also available with a 2.9 ratio, and is nearly as light as the weak suck 7 1/4.
 
My Dart lite has a November of 75 build date, very early model with non std "lightweight" options.
I have the A body 4 speed OD trans so that will be used.
Thanks for the input. When I get there I will try both 3.23 and 3.55 since I have them to see which is better.
 
Backtobasics. Your overdrive is .69 (or 31% od). Knowing this, a 3.55 in
od is the eqivolent to a 2.44. If you put in a 2.76 rear, in od it would be a
1.90. 4.10 in rear would be 2.82 in od. Do the math to figure out what you
like without guessing. Hope this helps.
 
I have to second what Orv says.

I have an 833OD from a 75 Dart Sport, my OD ratio is .73

Running a 4.10 rear gear makes my car very quick off the line... and when you go into OD it is just a shade under a 3.00 gear ratio... decent for freeway cruising.

Besides that... turning a ratio like this saves the 833OD from being "torn up" because it no longer "lugs" in OD.

It seems as though the factory gear ratios originally used in the cars equipped with the 833OD were installed for strictly mileage purposes... and the "lugging" experienced while in OD was the cause for these transmissions getting a bad rap, because the stress was too great for the aluminum housing construction. Using a 4.10 (or a 3.23, 3.55, 3.73 or 3.91) vs a 2.45 gear makes a world of difference in these transmissions reliability.

You can also have the aluminum case bushed so it will not wear.
 
TheDemonator said:
I have to second what Orv says.

I have an 833OD from a 75 Dart Sport, my OD ratio is .73

Running a 4.10 rear gear makes my car very quick off the line... and when you go into OD it is just a shade under a 3.00 gear ratio... decent for freeway cruising.

Besides that... turning a ratio like this saves the 833OD from being "torn up" because it no longer "lugs" in OD.

It seems as though the factory gear ratios originally used in the cars equipped with the 833OD were installed for strictly mileage purposes... and the "lugging" experienced while in OD was the cause for these transmissions getting a bad rap, because the stress was too great for the aluminum housing construction. Using a 4.10 (or a 3.23, 3.55, 3.73 or 3.91) vs a 2.45 gear makes a world of difference in these transmissions reliability.

You can also have the aluminum case bushed so it will not wear.
Click to expand...

Anytime you transfer torque through the countergear and not straight through the transmission by coupling the main drive gear (GM terms Chrysler terms eludes me at the moment) to the mainshaft (output in this case), the countergear shaft will load against the transmission case. Unbushed aluminum cases generally cannot stand up this over time and the countershaft hole(s) in the case wallows out or elongates (Muncie four speeds with an aluminum case are renowned for this). Because the OD transmission gear set exchanges third for fourth, anytime you are in high gear the countergear is loaded, causing the OD gearset transmission to wear the unbushed aluminum case seemingly faster than a straight four speed non OD transmission. Switching to an iron case, having the case bushed, or switching to a bushed aluminum case (Passon Performance, perhaps others) should minimize or eliminate the problem.
 
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