differencial ratios

[wisbangman]

Help ! - I just bought a 1939 ford. I would like to put a 3.54 rear end in it . Could some one tell me how to determine my current ratio ? I read in the 1938/39 book that there are stampings on the differencial housing that will tell what is in the car, anyone know about this ? I also read that there are 6 and 10 spline units - any info on what a 39 ford would have ? Any one have one for sale ?...thanks, barners

[41ford1]

The teeth count of the ring and pinion are stamped on the bottom differential housing near the pinion. To get the ratio divide the larger by the smaller. These numbers are only good if the factory gear set was not changed. To find what is really in the car. Jack a rear wheel off the ground. Put the trans in high. Turn the engine BY HAND and IGNITION OFF. Count the number of engine revolutions it takes to make the wheel turn 2 revolutions. That will be your ratio.

[ford38v8]

Wisbang, 72 years is a long time for any drivetrain to remain original, so the advice you've had is good, you should determine your ratio by the mechanical method rather than to assume the gears are original to the banjo. A hint on that: Do the revolution thing X 10, as the result will be much more accurate, the ratios being so close to each other. Also, the reason, you may have wondered, "2 revs of the wheel", is that the spider gear will multiply the revs X 2 because the other wheel is still on the ground.

Someone else advised that you can drain the diff, mark the ring gear with chalk or red paint or something, and count the teeth. I don't know if this is possible from the fill or drain hole, but worth a look-see!

To determine if you have a solid or hollow driveshaft, look for the zerk fitting at the center of rthe torque tube. The solid shaft requires a center bearing, the hollow does not.

[Kurt in NJ]

The easy way--- put in 3rd gear(direct) with the valve stem down (easier to see), put a piece of masking tape(or whatever you have) on the pulley, push car for 1 revolution of wheel while counting crankshaft revolutions ----if it stops a little more than 4 rev--4:11, if it does 3 1/2 ---3:54 -,-3 3/4 --3:78 even if the pulley is greasy the tape will stick to where the belt rides and you have a little "flag" to watch --if the front and rear tires are the same size you only have to feel the valve stem as you push

[Hoop]

This is an absolutely accurate way to determine the ratio. It's very easy, .... but I'm not selling anything. Everyone is welcome to try it their own way.

You do not have to drain the oil. You use white paint. I've done it many times.

Here's what I posted available in SEARCH:

"A 3.78 ring gear has 34 teeth. The 4.11 has 37, and the 3.54 has 39. (One of the numbers stamped under the housing.)

It's very easy to simply count the teeth on the ring gear to confirm the number. "

Here's the long explanation:

Jack up the rear.

Remove the fill plug from the banjo. Place a blob of white paint on one of the gear teeth that you cleaned off. (You do not have to drain the oil, which it probably needs anyhow, but it may make it easier.)

Reach over and turn the wheel with your toe (one man job) as you "feel" and count the teeth with the tip of a screwdriver ... while watching with a flashlight for the white blob to reappear.

[wisbangman]

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Hi guys !.... and thanks for the advice !!! The method of marking the teeth with white paint, and counting them, is briiliant ! When you say there is a number marked under the housing, { I haven't crawled under yet } It would sound too simple to have the number marked, be the actual ratio. Is that the number, or is the number the amount of teeth ? Also, it would seem that there would be a progression of fewest teeth would equal highest numeric ratio, Highest amount of teeth would equal lowest numeric ratio. or vice-versa. It seems that the numbers quoted, don't follow a progression but honestly, what do I know ? I'm the guy looking for the help ! Could you clarify this one for me?....and again....thanks !

[Hoop]

"Also, it would seem that there would be a progression of fewest teeth would equal highest numeric ratio, Highest amount of teeth would equal lowest numeric ratio."

There are TWO gears, a ring and a pinion.

"It would sound too simple to have the number marked, be the actual ratio. Is that the number, or is the number the amount of teeth ?"


The stamped numbers are the actual tooth count for each of the two gears. Divide the smaller number (pinion) into the larger number (ring).

The combinations are 9-34. 9-37 and 11-39.

Do the math.

[wisbangman]

Two more teeth on the 3.54 pinion ! now it makes perfect sense. Thank you very much !!! for the expert advice, You really know your stuff and I REALLY am greatfull.....tom

[Hoop]

"You really know your stuff "

No, I know a little stuff. There are guys on the Ford Barn that know LOTSA stuff.

Really I only use my technique because it gives me an excuse to crawl under the car. To me, guys who crawl under cars seem to be doing something slightly more important than guys bent over a front fender with their heads under the hood.

Both very impressive however. Like my post, it makes you think they know what they're doing.

( ... thanks.)

[wisbangman]

A little more help, if you would. And thanks in advance !!! - Now I know where the identifing marks are, what they mean, and how to determine if the teeth count is right. Now I need to know if the 6 spline or 10 spline info in the 38/39 ford book is a factor, and what my options are if I find a 3.54 with the spline number that does not match mine ? What is the proper way to determine which spline count my car has ? What years will work as a doner to my car [ a 39 standard ] And last, what Is the best way to accomplish this, by pulling the axels and replacing the center section or ? And if anyone has a 3.54 they want to part with, please give me a call at 920-410-4866

[JM 35 Sedan]

wisbangman, Your '39 should have a solid drive shaft inside the torque tube (unless someone changed it over the years). The solid drive shaft will have a center bearing mounted in the torque tube and will also have a zerk grease fitting on the outside of the torque at about center. See if you have that zerk fitting. The solid drive shaft will have six splines on the pinion end. There would be a six/six splined female coupler/adapter pinned to the drive shaft and the six spline pinion. A six spline R&P set will drop right in and if you happen to locate a ten spline R&P set, it will also work after you purchase a six/ten splined female coupler/adapter. Good luck!

[wisbangman]

Thanks to every one for the great advice !!!! What a great bunch...I'm sure glad I didn' buy a Chevy !

[Straightpipes]

Heres how to tell what gears tou have in a Ford flathead..... Get out on the street in front of your house, Wind her up to around 3 grand in first and dump the clutch.

If she smokes the tires shes 4:44
If she peals out shes 4:11
If she chirps the tires shes 3:78
If she lugs a little and possibly burns the clutch shes 3:54

I goota stop drinkin this stuff while on the Computzer,...

[38 coupe]

If you are going to change the ring and pinion, why not go to a 3.25 ratio? Assuming a 28 inch tall rear tire, a 1:1 high gear in the transmission, and a 3.78 axle ratio you are turning a bit over 2700 r.p.m. With a 3.54 ratio at the same r.p.m. you now go 64 m.p.h. This isn't a lot of difference. With a 3.25 ratio at the same r.p.m. you now go 70 m.p.h. This seems like a lot more gain for the trouble to me.

[Straightpipes]

Originally Posted by 38 coupe

If you are going to change the ring and pinion, why not go to a 3.25 ratio? Assuming a 28 inch tall rear tire, a 1:1 high gear in the transmission, and a 3.78 axle ratio you are turning a bit over 2700 r.p.m. With a 3.54 ratio at the same r.p.m. you now go 64 m.p.h. This isn't a lot of difference. With a 3.25 ratio at the same r.p.m. you now go 70 m.p.h. This seems like a lot more gain for the trouble to me.

If you put a 3:25 ratio in a stock or near stock 39 Ford you will be burning the clutch trying to get the thing moving. Henry put a 3:78 in there for a reason. Bad enough with a 3:54.
Why does everybody need to go 70 MPH with an old Ford which was not built for that speed? If you want to go fast put a different engine in it......

[wisbangman]

My intention is to drive the car at 55 or tops 60 miles per hour. Henry never desiged them for that. Roads in the 30's / 40's were pretty much 50 and under. I believe that during ww 2 there was a 45 mile per hour nation wide speed limit in force. We are talking about cars that weigh more than a lot of modern cars, and have 1/2 the horsepower of modern cars, trying to keep up on todays roads with todays traffic [ the speed limit says 55 you do 60, and everyone passes you ! Any way I got some first rate ideas from everyone....thanks

[CBoz]

My Zephyr came stock with a 4.44; too low for even comfortable around-town driving IMHO. I have a 4.11 now, and I find that it's just fine, and I don't have to slip the clutch coming out of first. Any higher however, and it would be a little slow taking off. My car is a good deal heavier than a Ford (especially with my fat a** in it), so you can probably get by with something higher, but I can say that with a 4.11, 55 or 60 is not unreasonable. You will be winding up it some, but flatheads actually like it.