The Ford Barn

rfitzpatrick · 02-18-2022, 12:27 PM

I'm having to replace my rear end, it's gear ration: 3:78. It's a '31 PicUp I dearly enjoy driving, live in a small South Fla. city. It's driven semi-daily. I'm considering going 3:54 don't want the 4:11 at all. I'd like some advise/experience from you guys. Altho traffic has gotten rude lately, my 3:78 could keep up
Thanks

Patrick L. · 02-18-2022, 12:46 PM

There is about 4 mph difference between the different ring/pinions. A 3:25 [?] is about the same.

eagle · 02-18-2022, 03:56 PM

A pickup is a light weight A. The lower gear ratio should work great for around town. If you're going to start hauling grain and gravel, 4:11 would be the way to go.

Synchro909 · 02-18-2022, 05:23 PM

The convention is that the lower ratio actually has a higher number the way it is expressed. 4.11 is a lower ratio than 3.78. That is, for the same engine revs, the speed is lower.

Terry, NJ · 02-18-2022, 08:28 PM

My Coupe needed a rear! Period! I had two choices, A) Buy a good used rear, or B) rebuild the old rear with a new gear set. I chose B. Rebuilding A differentials is something I never did before. First off, Get the new gear set I chose 3.54 with bearings etc, about $700. This coupe is not a daily driver and the whole project took about 2 months. Best thing I ever did! Better Gas mileage, a lot quieter, but the rear surprise was the better hill climbing. I have a hill nearby and the old rear couldn't make it better than 35 (Traffic was usually about 50 mph)with the new rear, I make it at 40/45. The 3.54 seemed to unlock the power (torque) of the engine. The only mod is a bored out manifold with B carburetor. I heartily recommend you do this.
Terry

GPierce · 02-18-2022, 08:48 PM

Sponsored Links (Register now to hide all advertisements)

My CCPU has a 3.54. It’s more than adequate. Toured West Virginia and the Utah/Arizona National parks with no issues. Drove from the North Rim of the Grand Canyon to Kanab,UT at 50+ mph.

nkaminar · 02-19-2022, 07:30 AM

I would think that the 3.54 would be adequate for Florida where there are no mountains to climb. It also depends on your engine. How is the torque? Is the engine a strong runner?

Terry, NJ · 02-19-2022, 09:54 PM

My advice! Find Tom Endy's site on rears and use it for a guide. It details all the little stuff you will need to complete the job. Also I recommend you purchase a laser thermometer/heat gun so you can monitor the heat rise of the parts once the car is back up and running again. they're hand when doing a brake job too. In fact, they can be a wonderful diagnostic tool that every mechanic should have in his box. For engines, radiators, transmissions , wheel, and rear ends. Good luck!
Terry

Bob Bidonde · 02-20-2022, 08:25 AM

Here is a performance chart for various gear ratios:

rfitzpatrick · 02-20-2022, 09:39 AM

Bob.
Now I didn't know about the 3.70. I assumed, by rotating my rear tire in gear, with the hand crank that I was 3.78.
I'm keeping your placard on my phone
Thanks Everyone

tinkirk · 02-20-2022, 10:30 AM

Quote:

Originally Posted by rfitzpatrick

I'm having to replace my rear end, it's gear ration: 3:78. It's a '31 PicUp I dearly enjoy driving, live in a small South Fla. city. It's driven semi-daily. I'm considering going 3:54 don't want the 4:11 at all. I'd like some advise/experience from you guys. Altho traffic has gotten rude lately, my 3:78 could keep up
Thanks

Not knowing what you have for a motor but ,i can tell you i have a 3:25 gear and would not want anything lower, if they made a 3;00 gear i would go with that now
it is your car your choice

Terry

Bob Bidonde · 02-20-2022, 12:06 PM

rfitzpatrick, I think that the 3.70:1 gears were aftermarket when the Model A was new. I do not know if any exist today.

johnneilson · 02-20-2022, 12:14 PM

Bob,
it might be helpful to show the tooth counts on the R&P for each ratio.
J

Tom Endy · 02-20-2022, 01:14 PM

Quote:

Originally Posted by Bob Bidonde

rfitzpatrick, I think that the 3.70:1 gears were aftermarket when the Model A was new. I do not know if any exist today.

See attached link

http://www.santaanitaas.org/wp-conte...ing-Pinion.pdf

Tom Endy

Tom Endy · 02-20-2022, 01:17 PM

Quote:

Originally Posted by Terry, NJ

My advice! Find Tom Endy's site on rears and use it for a guide. It details all the little stuff you will need to complete the job. Also I recommend you purchase a laser thermometer/heat gun so you can monitor the heat rise of the parts once the car is back up and running again. they're hand when doing a brake job too. In fact, they can be a wonderful diagnostic tool that every mechanic should have in his box. For engines, radiators, transmissions , wheel, and rear ends. Good luck!
Terry

See attached link.

http://www.santaanitaas.org/wp-conte...vised-2014.pdf

Tom Endy

johnneilson · 02-20-2022, 03:09 PM

Thanks Tom,

very helpful

John

02-18-2022, 12:27 PM	#1
rfitzpatrick Senior Member Join Date: May 2018 Location: Coral Springs, Florida Posts: 551	Differential Gear Ratio I'm having to replace my rear end, it's gear ration: 3:78. It's a '31 PicUp I dearly enjoy driving, live in a small South Fla. city. It's driven semi-daily. I'm considering going 3:54 don't want the 4:11 at all. I'd like some advise/experience from you guys. Altho traffic has gotten rude lately, my 3:78 could keep up Thanks

02-18-2022, 12:46 PM	#2
Patrick L. Senior Member Join Date: May 2010 Location: Largo Florida Posts: 7,225	Re: Differential Gear Ratio There is about 4 mph difference between the different ring/pinions. A 3:25 [?] is about the same.

02-18-2022, 03:56 PM	#3
eagle Senior Member Join Date: Aug 2011 Location: Eagle Bend, MN Posts: 2,025	Re: Differential Gear Ratio A pickup is a light weight A. The lower gear ratio should work great for around town. If you're going to start hauling grain and gravel, 4:11 would be the way to go. __________________ "There are some that can destroy an anvil with a teaspoon and shouldn't be allowed to touch anything resembling a tool."

02-18-2022, 05:23 PM	#4
Synchro909 Senior Member Join Date: Jun 2014 Location: Melbourne, Australia Posts: 7,496	Re: Differential Gear Ratio The convention is that the lower ratio actually has a higher number the way it is expressed. 4.11 is a lower ratio than 3.78. That is, for the same engine revs, the speed is lower. Last edited by Synchro909; 02-18-2022 at 06:02 PM.

02-18-2022, 08:28 PM	#5
Terry, NJ Senior Member Join Date: May 2010 Location: Bucks Co, Pa Posts: 3,740	Re: Differential Gear Ratio My Coupe needed a rear! Period! I had two choices, A) Buy a good used rear, or B) rebuild the old rear with a new gear set. I chose B. Rebuilding A differentials is something I never did before. First off, Get the new gear set I chose 3.54 with bearings etc, about $700. This coupe is not a daily driver and the whole project took about 2 months. Best thing I ever did! Better Gas mileage, a lot quieter, but the rear surprise was the better hill climbing. I have a hill nearby and the old rear couldn't make it better than 35 (Traffic was usually about 50 mph)with the new rear, I make it at 40/45. The 3.54 seemed to unlock the power (torque) of the engine. The only mod is a bored out manifold with B carburetor. I heartily recommend you do this. Terry

02-18-2022, 08:48 PM	#6
GPierce Senior Member Join Date: Dec 2014 Location: Germantown,TN Posts: 516	Re: Differential Gear Ratio Sponsored Links (Register now to hide all advertisements) My CCPU has a 3.54. It’s more than adequate. Toured West Virginia and the Utah/Arizona National parks with no issues. Drove from the North Rim of the Grand Canyon to Kanab,UT at 50+ mph.

02-19-2022, 07:30 AM	#7
nkaminar Senior Member Join Date: Jan 2012 Location: Western North Carolina Posts: 3,900	Re: Differential Gear Ratio I would think that the 3.54 would be adequate for Florida where there are no mountains to climb. It also depends on your engine. How is the torque? Is the engine a strong runner? __________________ A is for apple, green as the sky. Step on the gas, for tomorrow I die. Forget the brakes, they really don't work. The clutch always sticks, and starts with a jerk. My car grows red hair, and flies through the air. Driving's a blast, a blast from the past.

02-19-2022, 09:54 PM	#8
Terry, NJ Senior Member Join Date: May 2010 Location: Bucks Co, Pa Posts: 3,740	Re: Differential Gear Ratio My advice! Find Tom Endy's site on rears and use it for a guide. It details all the little stuff you will need to complete the job. Also I recommend you purchase a laser thermometer/heat gun so you can monitor the heat rise of the parts once the car is back up and running again. they're hand when doing a brake job too. In fact, they can be a wonderful diagnostic tool that every mechanic should have in his box. For engines, radiators, transmissions , wheel, and rear ends. Good luck! Terry

02-20-2022, 09:39 AM	#10
rfitzpatrick Senior Member Join Date: May 2018 Location: Coral Springs, Florida Posts: 551	Re: Differential Gear Ratio Bob. Now I didn't know about the 3.70. I assumed, by rotating my rear tire in gear, with the hand crank that I was 3.78. I'm keeping your placard on my phone Thanks Everyone

02-20-2022, 12:06 PM	#12
Bob Bidonde Senior Member Join Date: May 2010 Location: Long Island, NY Posts: 3,464	Re: Differential Gear Ratio rfitzpatrick, I think that the 3.70:1 gears were aftermarket when the Model A was new. I do not know if any exist today. __________________ Bob Bidonde

02-20-2022, 12:14 PM	#13
johnneilson Senior Member Join Date: Jun 2010 Location: 34.22 N 118.36 W Posts: 1,054	Re: Differential Gear Ratio Bob, it might be helpful to show the tooth counts on the R&P for each ratio. J __________________ As Carroll Smith wrote; All Failures are Human in Origin.

02-20-2022, 03:09 PM	#16
johnneilson Senior Member Join Date: Jun 2010 Location: 34.22 N 118.36 W Posts: 1,054	Re: Differential Gear Ratio Thanks Tom, very helpful John __________________ As Carroll Smith wrote; All Failures are Human in Origin.