Rear end gear ratio

[mike gaines]

Its been some time sice I last posted on the site. You may remember I had endless difficulties trying to steer my 58 Silverhawk. Replaced almost every front suspension and steering part without success. Eventually removed the Kingpins , Everything was good , including the upper and lower brass bushes!!! Lower brass bushes , I hear you say!! Yes , some twit had used a bush instead of the needle roller bearing. When the car was jacked up , the front turned beautifully but this was not the case when the pins took up the weight of the car when it was dropped off the stands.New roller bearings and all is perfect , a very expensive excercise.
I need some help on rear end ratios.The car has a 3 speed manual OD gear box and the tag on the diff has these markings - 41-11 and 3-73. I presume the ratio is 3-73 but what does the other marking relate to ? I want to install an auto Turbo 400 3 speed gearbox and the 3-73 will be too highly geared for highway cruising. I dont know whether this is a type 27 gearbox ( how would I find this out ). Can I reduce the ratio to 3-07 or 3-31 by simply replacing the crown gear and pinion ? What else should I know ?

[Skip Lackie]

41 and 11 are the number of teeth on the ring and pinion. 41 divided by 11 equals 3.73. The axle model should be cast into the lower right of the back side of the pumpkin.

Skip Lackie
Washington DC

[Guest]

if you run a 27 inch tall tire and keep the 3.73 60 mph will about 2800 rpm. 70 mph will be about 3250. it will be a big difference that what you are use to with the over drive.
to change your gear ratio you would have to ghange your ring and pinion. it isnt a bad job but if you have never done it before i would suggest finding someone who has or a have a shop do it, so it is set correctly. if you are wanting to cruise down the highway at 70 you probably go with the 3.07 rpm would be about 2700 at 70. there are other varibles trany slip and exact tire size. the 400 is a good transmission but it is a little over kill for a stock stude let alone a built one, the rob more power are heavier and not much stronger than a 350 turbo or 700 r4. the best thing to do if you are going to mark the gm automatic swap is to go with the 700r4 automatic with over drive you could leave your gear and still cruise nice and with the low 1st gear of the 700r4 it would jump from a dead stop very well.
i hope this helped some
let me know what you do and how it comes out.
also i used a steel tech adapter when i did mine and had alot of trouble with it. i have never used another brand, but someone in this forum has bound to used a diffent one.

Erin Hays
"From Stuck and Rusty to Slick steel and sex appeal"
RZRECTD
1961 Hawk
1962 Lark
1963 Wagonaire

[mike gaines]

Thanks for the replies , but as always happens , I have now been offered a recently redone chevy 400 motor with turbo 350 box. Aside from the originality aspect , do you have any idea what the rear end ratio would need to be to provide reasonable pull off and comfortable highway cruising. My tire diameter is 26.5 inches.

[Guest]

if you are going to run the 400 engine pick a gear. it will have plenty of turque to do most anything you want to do. if you are planning on cruising down the highway alot and racing very little i would look between a 3.23 to 2.73. i have a friend with a 2.73 gear in a camaro and still turns mid 12's on motor. he has some fun getting it off the line sometimes but he likes it. i had a 3.23 in my old ford and it did well running down the road and still got off the line pretty good.
here is a formula that might help you decide.


RPM @65mph (Derived from Max MPH formula)
Formula: RPM = mph x gear x final ratio x 336/tire diameter
Example: RPM at 65mph
65mph x 0.864(5th gear) x 3.54(diff gear) x 36/24.0"(tire dia.) = 2783rpm

Erin Hays
"From Stuck and Rusty to Slick steel and sex appeal"
RZRECTD
1961 Hawk
1962 Lark
1963 Wagonaire

[PackardV8]

Hi, Mike,

With the 400" SBC, you will have all the torque you need to pull whatever gear you want. A 2.88 would be a good cruising gear if you plan lots of highway miles. The 3.73 is for performance work.

Unless someone is GIVING you the TH350, consider going with a 200-4R overdrive and keeping your 3.73 rear gear. That combo would make a great street driver. Think about it; GM wouldn't have spent the money if the four-speed-overdrive automatic wasn't a quantum improvement over the already proven TH350.

The rear axle model designations are cast into the housing. If your rear axle is a Type 27 and not a 44, you will have to drive it sanely for it to survive. An aside not generally understood is the 3.73 will last longer than the 2.88, as the higher torque multiplication will spin the tires sooner and releave the torque pressure more readily than the 2.88.

If you plan to ever run it hard or take it to the track, a Dana 44 is necessary, inexpensive and bulletproof.

Bottom line any combination heretofore discussed will work, just some better than others for some uses.

thnx, jv.

PackardV8

[Hippie]

quote:Originally posted by PackardV8

Hi, Mike,



Unless someone is GIVING you the TH350, consider going with a 200-4R overdrive and keeping your 3.73 rear gear. That combo would make a great street driver.

You said it! [8D] A friend of mine owns a transmission shop and builds a lot of 200-4R's for rods and muscle cars. It is a really sweet setup with 3.73's and with a few upgrades can take a lot of HP and TQ. If I decide to ditch the stock drivetrain in my Stude I won't run anything but a 200-4R.

Analog man in a digital world.

[Guest]

i like the 700r4 better for the biggest reason of the 3.13 first gear ratio. it lets you run a higher gear but come out of the hole strong.

Erin Hays
"From Stuck and Rusty to Slick steel and sex appeal"
RZRECTD
1961 Hawk
1962 Lark
1963 Wagonaire

[mike gaines]

Thanks for all the responses , but I need a bit more help.The motor comes with the turbo 350 box, so would be stuck with it.Secondly , in trying to understand the formula - rpm = mph x gear x final ratio x 36/tire diameter, what does "gear " relate to ?
Lastly , will a crown gear and pinion from a type 27 diff fit a dana 44 diff , which is in my car ?

[Alan]

Gear is the transmission ratio. On the 350 it is 1:1 On your overdrive it is .72. And lastly No.

[Guest]

the gear your ratio your transmission is in for drive it would be 1. overdrive maybe .7 second gear 1.54 maybe.
if you are just worring about cruising speeds use 1 since you have no overdrive.
i noticed a type o in the formula so re wrote it. twice

rpm = mph x gear x final ratio x 336/tire diameter

Formula: RPM = mph x gear x final ratio x 336/tire diameter
Example: RPM at 65mph for stock 280Z 5spd
65mph x 0.864(5th gear) x 3.54(diff gear) x 336/24.0"(tire dia.) = 2783rpm

Erin Hays
"From Stuck and Rusty to Slick steel and sex appeal"
RZRECTD
1961 Hawk
1962 Lark
1963 Wagonaire

[mike gaines]

Again , many thanks , but , finally ....... at what RPM would the 400SBC be comfortable , cruising at 75MPH ?
I have mailed Dana to see what crown gear and pinions they can supply as neither SASCO nor SA have new parts.

[Guest]

SBC wheather a 400 350 327 or 283 dont seem to mind cruiser upwards of 3000 rpm allday long. i would shot for around 2400, but if you combinations ends up at 2800 you will have no drivability problems just might pull down mpg some. a 3.23 would be about 75 @ 3000 rpm. so i guess you are looking at a 3.23 gear and down i am not sure on the dana options but some ratios are offered as high as 2.40. you might look at a 3.08 or a 2.73.

Erin Hays
"From Stuck and Rusty to Slick steel and sex appeal"
RZRECTD
1961 Hawk
1962 Lark
1963 Wagonaire

[PackardV8]

Hi, Mike,

As always, your car, your money, your choice. Since you asked for advice, just a couple of points of clarification:

1. Since it is all Chevy, you are not "stuck" with the TH350. There is always the option to sell it to a racer and buy an overdrive transmission which bolts right on. I personally would choose a built 200-4R. The 400" SBC has more than enough torque and will like the closer ratios of the 200-4R, as well as its taller overdrive.

Agree, if it were a smaller Stude V8 one may appreciate the lower first gear of the 700R4. I have one in my Avanti R1.

2. You don't need to go directly to Dana. Any local 4X4 shop, any Jeep dealer, will have the parts and know how to change gears in a Dana 27 or 44. These are the most widely used rear axles in the history of the automobile. Studebaker, Jeep, Hudson, Jaguar, Sunbeam, Ford, Chevy, International, and many others used them.

thnx, jv.

PackardV8

[laughinlark]

Hey Mike
For gears, go to http://www.ringpinion.com/
They are fimilar with Studies.

Gordon