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Old 09-03-2015, 05:51 PM   #1
1955cj5
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Default 4:11 ratio vs. 3:78

Through another thread on this site I learned that my differential ratio is 4:11. Remember I also have a 4-speed which does not really matter in high gear but may affect the lower gears.

I also learned that my speedometer is reading about 5 mph high, so when cruising happily at 40 I'm really doing 35. I think it's the driven gear but that is another story(number worn off the turtle)...when I indicate 45 or so it starts to sound a little over-worked. I have been as high as 50 indicated, but don't plan to do that again!

I found a complete rear end out of a 31 Tudor, torque tube, brakes wheels spring etc. Assuming it's a 3:78 would the different ratio make the truck any more drivable or would I hardly notice the difference? Would it be worth the effort? I have no plans for touring.

Would a 3:54 be too high and just create other problems?

Thanks,

Randy
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Old 09-03-2015, 06:46 PM   #2
BRENT in 10-uh-C
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Default Re: 4:11 ratio vs. 3:78

The difference between the 4.11 and 3.78 is about 3-4 mph. A 3.54 should be more than adequate and add about 6 mph over what you presently have.
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Old 09-03-2015, 07:31 PM   #3
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Default Re: 4:11 ratio vs. 3:78

i have 3.54s in both my a's. except for a long hill near here, i don't notice the difference and they do slow the engine down, which was the objective. a couple mos ago i inadvertently got on the interstate (long story) and was forced to pound the old roadster at 60-65 in heavy traffic for several miles. i'm sure the gears helped, and i damn sure do not intend to do that again.

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Old 09-03-2015, 08:46 PM   #4
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Default Re: 4:11 ratio vs. 3:78

3.78 + 21% OD is just about perfect, for me anyway.
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Old 09-03-2015, 08:50 PM   #5
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Default Re: 4:11 ratio vs. 3:78

I looked at overdrives...have one on the jeep...it would be a nice accessory for sure....but with a pending engine overhaul just too many $$...
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Old 09-04-2015, 08:06 AM   #6
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Default Re: 4:11 ratio vs. 3:78

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I installed a 3.54 in my '28 CCPU about ten years ago and find it works quite well. Nothing special about the engine I am running in the PU. Plenty of power on the hills.
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Old 09-04-2015, 04:21 PM   #7
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Default Re: 4:11 ratio vs. 3:78

Here is how to figure how fast you will go for different gear ratios.
Engine RPM, DIV BY GEAR RATIO EQUALS RPM OF REAR WHEEL, TIMES CERC OF REAR WHEEL EQUALS IN PER MIN, DIV BY 12 EQUALS FT PER MIN , TIMES 60 equals FT PER HR DIV 5280 equals MPH.
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Old 09-04-2015, 07:13 PM   #8
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Default Re: 4:11 ratio vs. 3:78

So, working that backwards, at 40 mph I'm turning about 1940 rpm with that 4:11 rear end.

With a 3:54 rear end 40 mph would need about 1670 rpm..

1940 rpm for a sustained period of time seems high..
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Old 09-05-2015, 12:38 AM   #9
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Default Re: 4:11 ratio vs. 3:78

I have changed a few Model A trucks over from the 4:11 to 3:78 and 3:54 and the owners have always been happy with the change because of increased speed with lower rpm.
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Old 09-05-2015, 01:36 PM   #10
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Default Re: 4:11 ratio vs. 3:78

I had a 47 Mercury Convertible with a 4.44 rear end once. It was a "screamer"! It was horrible to drive! The engine was always overreving! Put the higher rear in it!
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Old 09-05-2015, 01:51 PM   #11
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Default Re: 4:11 ratio vs. 3:78

I agree with Brent... it should handle the 3:54 just fine and be very comfortable on the road. Unless you have a worn out tired engine ( which I'd fix if that were the case)
The 3:54 makes them cruise more easily.

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Old 09-05-2015, 05:05 PM   #12
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Default Re: 4:11 ratio vs. 3:78

Here are the speeds you will go at a comfortable 1750 RPM with 21 inch wheels. 95 inch circumference.
4.11 = 38.3 MPH
3.78 = 41.6 MPH
3.54 = 44.4 MPH
3.78 with a Mitchell overdrive = 56 MPH
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Old 09-05-2015, 05:29 PM   #13
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Default Re: 4:11 ratio vs. 3:78

Just curious what the 3:26 would give you? I would think you would need a balanced crank and cam and possibly a better head.
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Old 09-05-2015, 05:55 PM   #14
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Default Re: 4:11 ratio vs. 3:78

At 1750 RPM a 3.26 would give you 48.2 MPH
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Old 09-05-2015, 07:42 PM   #15
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Default Re: 4:11 ratio vs. 3:78

I'm wondering, if you're not planning on touring, what is the plan?

If parades or hill country is your thing, the 4.11 will be great. My mom's coupe has the 4.11; it's great for parades but getting from town to town is not so nice. If you want to drive on highways (but not interstates) without folks running up your back and without roaring around at high rpms, swap it out.

For years, many folks said that the 4.11 in combination with an overdrive, was the ideal combo. I never understood that; it seemed to me that they undo each other so that you were basically back where you would be with the common 3.78 rear end and no OD.

I've got an F-150 4-speed/OD with 3.78 rear end and 5.9 cr head. Both 1st and 2nd are too slow--I'm not pulling stumps! I wish I had a 3.54 rear end.

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Old 09-06-2015, 07:56 AM   #16
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Default Re: 4:11 ratio vs. 3:78

Quote:
Originally Posted by wmws View Post
Here are the speeds you will go at a comfortable 1750 RPM with 21 inch wheels. 95 inch circumference.
4.11 = 38.3 MPH
3.78 = 41.6 MPH
3.54 = 44.4 MPH
3.78 with a Mitchell overdrive = 56 MPH
Thanks for this information. The Mitchell is available in 26% and 36% overdrive ratios. which ratio is used for this example? Thank you, Jeff
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Old 09-06-2015, 09:11 AM   #17
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Default Re: 4:11 ratio vs. 3:78

The example with the Mitchell is the 26%. I have a Tudor with 3.78 and the 26% Mitchell OD. I think it is ideal. I have it all figured mathematically on a spread sheet so I can just plug in any RPM and any ratio and get the MPH.
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Old 11-11-2015, 09:26 PM   #18
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Default Re: 4:11 ratio vs. 3:78

So,

I still question my rear axle ratio....it drives pretty nice at 35-40 mph, doesn't sound like it's working too hard..

Today I drained the differential, and then I counted the ring gear teeth....

34 teeth.....

I've found the tooth count listed somewhere but cannot find it now...

So does a 34 tooth ring gear mean I have a 4:11 or a 3:78? Or is it inconclusive?

I cannot count the pinion teeth easily at this point....
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Old 11-11-2015, 09:45 PM   #19
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Default Re: 4:11 ratio vs. 3:78

The question is not which is better...

The question is which do I have?

Bratton's lists the tooth count for all it's gear sets except for the 4:11....

I'm thinking I have a 3:78...

Because to get 4:11 with a 34 tooth ring gear you would have to have a 8.26 tooth pinion gear....not likely....

but a 9 tooth pinon gives you 3.78.....
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Old 11-11-2015, 09:48 PM   #20
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Default Re: 4:11 ratio vs. 3:78

Quote:
Originally Posted by forever4 View Post
see the link for all your ring and pinion ratio questions.

click> Comprehensive chart of Model A and B Ring and Pinion Axle Ratios

Well, there is the answer.....

With a 34 tooth pinion I must have a 3:78....no other option...

So now I question the "chalk mark at 12'oclock turn the engine twice ratio check".....
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