rear end ratio
Lead Driver
Joined: Oct 1999
Posts: 6,782
Likes: 558
From: Kalispell, MT
3.92 std or optional 4.27 to 1. There should be a metal tag under one of the bolts holding the back cover in place. 3.92 will read 47/12 and the 4.27 has 47/11 gears.
Easy way is to take the rear cover off and count the teeth on the pinion - either 11 or 12. The ring gear of both will have 47 teeth, naturally.
Easy way is to take the rear cover off and count the teeth on the pinion - either 11 or 12. The ring gear of both will have 47 teeth, naturally.
Lead Driver
Joined: Apr 2002
Posts: 8,813
Likes: 13
From: Littleton, New Hampshire
What kind of upgrades are you planning? If you plan to keep the original engine the rear you have might be preferable. If you use a newer engine and transmission you may want a taller rear gear. The exception to that would be if you used an OD transmission. Then the ratio you have may work out.
Vern
Vern
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Joined: Jan 2003
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From: 3rd Rock
Originally Posted by chapla
Does anyone know how I can find what my rear end ratio is in my 49 and 51 Ford f-1? Is one rear end preferred over other when upgrading truck? Thanks
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Elder User
Joined: Oct 2003
Posts: 785
Likes: 0
From: Canada
Rear End Ratios
Rear Diff Ratio check.
Jack, up one of the rear wheels, remove the drive shaft from the diff. Mark the tire, also the ground or pavement. Then have some one beneath the car “Use Jack Stand”, mark the diff yoke then rotate the wheel 20 times, slowly and have the person under the car count the revolutions of the diff. This will give you a pretty damm close rear end ratio if not accurate.
I have used this formula several times and it has turned out right on. Some say that you have to have both rear wheels turning, you do not. Also make sure you don’t talk while doing this as you can lose count easily.
Here is an example of what I am trying to tell you.
Turn the wheel 20 revolutions.
If the diff turns 35 ½ times you have a 3.54
If the diff turns 37 1/3 times you have a 3.72
If the diff turns 4 1/10 times you have a 4.10
If the diff turns 3 9/10 times you have a 3.91
Get the picture, believe me it works. Or if its a good cruiser you want without to much modifying, put in a 9 inch diff from a 67-72 Ford Pickup, you can use the brakes, the fit is great, and you have a wide selection of gearing. A friend of mine runs a 3.25 in his 50, all the engine is stock as well as the Tranny. The stock wheels even fit, as after 56 I believe the wheels changed for the 48-56 hub caps.
The choice is yours, there is a wide varity out there.
Jack, up one of the rear wheels, remove the drive shaft from the diff. Mark the tire, also the ground or pavement. Then have some one beneath the car “Use Jack Stand”, mark the diff yoke then rotate the wheel 20 times, slowly and have the person under the car count the revolutions of the diff. This will give you a pretty damm close rear end ratio if not accurate.
I have used this formula several times and it has turned out right on. Some say that you have to have both rear wheels turning, you do not. Also make sure you don’t talk while doing this as you can lose count easily.
Here is an example of what I am trying to tell you.
Turn the wheel 20 revolutions.
If the diff turns 35 ½ times you have a 3.54
If the diff turns 37 1/3 times you have a 3.72
If the diff turns 4 1/10 times you have a 4.10
If the diff turns 3 9/10 times you have a 3.91
Get the picture, believe me it works. Or if its a good cruiser you want without to much modifying, put in a 9 inch diff from a 67-72 Ford Pickup, you can use the brakes, the fit is great, and you have a wide selection of gearing. A friend of mine runs a 3.25 in his 50, all the engine is stock as well as the Tranny. The stock wheels even fit, as after 56 I believe the wheels changed for the 48-56 hub caps.
The choice is yours, there is a wide varity out there.
Postmaster
Joined: Mar 2000
Posts: 3,730
Likes: 13
From: Waynesville, OH
Welcome to the forum. 
As you've already seen, we have some friendly and helpful folks here.
Take a few minutes and read the sticky threads at the top of the forum, then set up your gallery with some pictures. We all love to see project trucks and any modifications you may make as you work.
As you've already seen, we have some friendly and helpful folks here.Take a few minutes and read the sticky threads at the top of the forum, then set up your gallery with some pictures. We all love to see project trucks and any modifications you may make as you work.
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New User
Joined: Oct 2005
Posts: 6
Likes: 0
Thanks for the responses. I am new to site and in the process of restoring a 49 Ford F-1. I have a 51 for donor. I want to swap out engine and transmission but keep original straight axle and rear end. Will the rear end hold up to a mid cubic size engine with moderate horsepower (300-350)? You mentioned using an overdrive transmission. I guess that is to keep rpms down. Thanks again
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Elder User
Joined: Oct 2003
Posts: 785
Likes: 0
From: Canada
Rear End Ratios
It would be much more simple to change the rear completely by puting in a 9 inch diff. The O/D installation is quite a setup to install, if you have never done it before, and costly.
The 9 inch diff will bolt right in and you have as I mentioned a wide range of gear ratios you can pick from to suit your needs. The nice thing about it is unless you get down and look at the diff you wouldn't even know there was a swap done.
If you have a 4.27 ratio now as I did in one of my trucks, you could probably burn rubber from a standstill in second gear. Also your engine would be running at a higher rpm at highway speeds, and I mean around the 50-55 mile per hr.
Some of these trucks had a 3.72 or 8 in them, they were not so bad. The one I have at present has a 4.10 in it, but it has the factory O/D, which helps out alot. Then again I only run around the tops 65 mph. Which ever way you go with this good luck and put some thought into it.
The 9 inch diff will bolt right in and you have as I mentioned a wide range of gear ratios you can pick from to suit your needs. The nice thing about it is unless you get down and look at the diff you wouldn't even know there was a swap done.
If you have a 4.27 ratio now as I did in one of my trucks, you could probably burn rubber from a standstill in second gear. Also your engine would be running at a higher rpm at highway speeds, and I mean around the 50-55 mile per hr.
Some of these trucks had a 3.72 or 8 in them, they were not so bad. The one I have at present has a 4.10 in it, but it has the factory O/D, which helps out alot. Then again I only run around the tops 65 mph. Which ever way you go with this good luck and put some thought into it.
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Lead Driver
Joined: Apr 2002
Posts: 8,813
Likes: 13
From: Littleton, New Hampshire
If you want to keep the original rear end it should hold up okay to an engine like you describe provided the rear is in good shape to start with. If you use something like a 302 and AOD that rear end should be fine. Yes, the reason for the OD transmission is to offset the high gears in the original rear. If you wanted to go with a non-overdrive transmission then you might want a taller rear gear. The nine inch from 57-72 is the standard replacement. 
Vern
Vern
Hotshot
Joined: Oct 2004
Posts: 15,882
Likes: 88
From: Durham NC
If your truck has the same spring spacing as the 53-56 the suggested replacement rear axle is a bolt in exchange, no mods required, and is the strongest that can be had (#1 choice for racing use), that's why it's being so highly recommended.
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