Transmission ratios 1947 4 speed
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Freshman User
Joined: Sep 2006
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From: Brunswick Maine
Transmission ratios 1947 4 speed
For my 1947 1 ton (79Y) , I am trying to calculate what gearing in a new ford 9 " will work best with a C-4 auto. I wanted to start by comparing to my stock rear and 4 speed manual ( with dual low) . (I think I may want something just a little taller than stock , which is good for about 55 MPH, flat out, as I will not be hauling any heavy loads) I have looked on the net and in old shop manuals for the gear ratios in the 4 speed but have not found anything. Can someone point me in the right direction ? Thank you.
Lead Driver
Joined: Oct 1999
Posts: 6,776
Likes: 553
From: Kalispell, MT
Here's some ideas to ponder - or not.....lol
Both your old 4 spd and the C4 have a final drive of 1:1.
Auto trans generally used with rear gears in the 2.75 - 3.25:1 range.
Most manual trans like 3.50 and up rear gears.
For what you described I'd find a rear with gears in the 3.25 - 3.50 range.
To answer your question, your current 4 spd gears should be like those shown in my 48 shop manual. 1 = 6.4, 2 = 3.090, 3 = 1.690, 4 = 1.0 and Rev is 7.820
Both your old 4 spd and the C4 have a final drive of 1:1.
Auto trans generally used with rear gears in the 2.75 - 3.25:1 range.
Most manual trans like 3.50 and up rear gears.
For what you described I'd find a rear with gears in the 3.25 - 3.50 range.
To answer your question, your current 4 spd gears should be like those shown in my 48 shop manual. 1 = 6.4, 2 = 3.090, 3 = 1.690, 4 = 1.0 and Rev is 7.820
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Freshman User
Joined: Sep 2006
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From: Brunswick Maine
Thank you very much. This is exactly what I was looking for. My owner 's manual says that I have either a 4.86 or 4.11 rear. I will jack up the rear and spin a wheel to verify which. To a novice it seems like quite a jump down to 3.50 which is why I've been a little unsure how to proceed. I will ,eventually, be modifying the (1953 8 AB) motor for a little more power ( mild upgrades) so more cruise friendly gears should be OK I think.
Elder User
Joined: Oct 2003
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From: Canada
I myself if you want to play safe and be able to haul, say a car trailer or something in that range. Go for the C-6 and 3.54 rears, the C-6 and the 9 inch are virtually indestructible and will give you good value and use out of your truck
Nice thing about the 9 inch diff is there are many gear options and you can experiment to get the one that best suits your needs.
Evem better you can if you wanted get rid of the 4 speed and go for 3 speed, and 3.25 rears and your off to the races. Makes for a good road gear and you can enjoy your truck on a trip.
Nice thing about the 9 inch diff is there are many gear options and you can experiment to get the one that best suits your needs.
Evem better you can if you wanted get rid of the 4 speed and go for 3 speed, and 3.25 rears and your off to the races. Makes for a good road gear and you can enjoy your truck on a trip.
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Freshman User
Joined: Sep 2006
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From: Brunswick Maine
Thank you. I have the C-6 / Ford 9" combo in my 46 Mercury convertible ( with a built 302) and as you say it has proven indestructible so far. An earlier post recommended the C-4 suggesting it as a better match for the low powered flathead. I will be making the decision on which drivetrain to purchase soon .The work will probably take place next winter now. Having too much fun driving on (very short) trips at the moment. Our Maine summer only lasts until August 15 anyway.
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