Transmission/differential question

[chrismfischer]

I have a beeeeeutiful 1946 1/2 ton dodge pickup. Inherited from Dad, who is passed, and taught youngest son to drive in it. He will get it when I croak. I've rebuilt engine, brakes, steering gear box, etc.... And it drives wonderful.

Only issue: has a 3 speed tranny and is so low geared that 50 mph feels like the Rpm's are excessive. I would really like to give it a lower gear ratio. What's the best way to accomplish this?

I'm thinking a new differential with lower ratio is the answer, but I don't know anything about diffs. Can someone explain what I would need to buy to make that happen? I can check the diff casting numbers if that helps. Im not sure it's the original rear axle to begin with. I'm hoping a new set of gears can be installed in the current axle. If so, is there a a place you can direct me to to acquire the gears please?

Btw, I don't want to drive the truck at high speeds. I just want to shift less and not over rev the I6.

Thanks! And please call me at 713 817 0528 if you want to discuss instead of doing the endless message thing. Chris Fischer in Kingwood, Texas

[Plymouthy Adams]

higher gear ratio will get you in the higher MPH with lower RPM but you did not state if you are wishing to stick with your current axle and change the gears or are open to a later model axle swap.....

[Young Ed]

The easiest swap is to find a car 3.90 or 3.73 and swap it in. Not 100% sure but it appears you might have 15s rather than the stock 16s. That is hurting your speed too. If you wish to keep the 15s I'd look for a 3.73. 

[BigDaddyO]

Nice looking truck.

[Don Coatney]

This might or might not help.

 

[MBF]

Not sure if the frame width is the same or not, but a friend of mine swapped in a rear from a Jeep Cherokee for road speed in his '38.   I don't remember if he had to move the spring perches or not, but it runs right along nicely with the original 218/3 spd, and he's got better brakes.     He also did a spindle swap on the front end with a late 60's/early 70's D100 that gave him self energizing brakes on the front end too.  Wheel bolt pattern were also the same.  That is a nice looking truck!   Good luck with your project.

[HotRodTractor]

That is a beautiful truck.

 

There are most likely 3 options:

 

1) Determine what gear ratio your rear end has - and then if you can get one that is numerically lower - you'll knock down some RPMs.

 

2) A very popular option is to swap the rear end for a newer unit and do some upgrades along the way (better brakes). For instance I just picked up a Mercury Mountaineer rear axle with 3.73 gears, limited slip diff, disc brakes, etc... for $200. I plan on cutting off all the brackets and welding on new to make it nearly a bolt in replacement as its nearly the correct width and the right bolt pattern.

 

3) Transmission swap - its fairly easy and cheap to install a T-5 transmission into these trucks giving you more gears and an overdrive. I think I have $100 into the S10 T5 that I have in the shop(S10 tailhousing puts the shifter in the right location, but there are better gear ratio sets available in other applications). You'll need to get an adapter setup (I think the one is $100 or less for everything on a 3 speed truck) plus clutch parts, driveshaft, etc...

 

 

I'm getting ready to do both option 2 and 3 on a Pilothouse truck. Its an ugly truck I bought for parts years ago and its grown on me.... I figure it will fit in with the current trend of patina and rat rod and quick, cheap build that I can have some fun with while I start shopping for a nice truck to make a daily driver out of..... and then I'll have my entire drive train already lined out.

[wayfarer]

As mentioned, you should start by determining the rear axle ratio. Anything lower than 3.5 will limit your speed...unless you like to hear the fan screaming....

 

Consider using a much larger tire/wheel. Plenty in the marketplace.

 

If you are considering a trans swap then this may be helpful:  http://p15-d24.com/topic/17737-5-speed-not-t5/

[chrismfischer]

Thanks everyone for your replies. I would like to keep everything as stock as possible, and I just redid brakes so they are fine. I kinda like having the old stuff the way it was.

So, I would prefer to keep the axle and change the gears if that is do-able. Btw, the new Coker tires are over sized and did help, but any larger would rub the fenders.

Please help me on these questions:

1. How do I determine the current diff ratio?

2. Exactly what parts would I need to rebuild the diff with lower gear ratio?

3. Any ideas on where I can find those parts?

Thanks again, and appreciate the compliments on the truck. It proudly displays a Go Mountaineers! Plate on the rear and a 1946 WVa license plate in honor of Dad who was a died in the wool West Virginian and Mountaineer grad.

Chris Fischer

[Plymouthy Adams]

look in and around the oil filler plug for the ratio to be stamped there....also I should be seen at the very top of the carrier assembly..can be expressed in ratio or gear count.......if you see 4.1 then ratio....if you see 41/10 then tooth count just do the division....

[William Davey]

Randy (owner of Randy's Ring and Pinion) told me that no new replacement differential gears are available for years prior to 1957. As suggested above, look for a used center section out of a 50's car that were available in 3:90 and 3: 73 ratios.  Or replace the entire rear axle assy with one from a Jeep Cherokee (search the forum).  

[Young Ed]

Here's my 46 getting a 3.90. Works out well with my 16s. They are also easier to find than a 3.73. You will have to pull brakes and axles then you can pull the center section and swap.

[DJ194950]

There are many different Mopar 3rd.member complete assemblies that Could interchange as a drop in 3rd. member with a different ratio if IF your original was not too low geared to start with, Can make a difference.

 

You need to find the ratio of what you have as mentioned before(location etc.) to be able to advise.

 

If you Find out what you have now, I will look in and Hollander Interchange manual to tell you what will Work. If you now have the normal (for that year) ratio, too many possibilities.

 

Many 39-56 Mopar rears May interchange, making finding other ratios avail. much easier to find in your area?  Most cars ran 3.9-3.73 and some Chrysler's used 3.5.

 

Let me know here if I can assist in this.

 

DJ

[chrismfischer]

Still trying to determine my differential gear ratio and type.

 

Ok, here's some pics of the differential (attached).  It's a Banjo type diff, which I read automatically makes it a 8 3/4" diff.  Has 6 nut/studs on driver side, 5 on the passenger side (11 total holding center unit in). 

Note the pics showing the casting numbers, also the number stamped on the top of the diff, which appears to be "354"  (?).

 

Can anyone tell me the gear ratio given this info?  

[JBNeal]

alternative differential identification procedure

[Phil363]

Still trying to determine my differential gear ratio and type.

 

Ok, here's some pics of the differential (attached).  It's a Banjo type diff, which I read automatically makes it a 8 3/4" diff.  Has 6 nut/studs on driver side, 5 on the passenger side (11 total holding center unit in). 

Note the pics showing the casting numbers, also the number stamped on the top of the diff, which appears to be "354"  (?).

 

Can anyone tell me the gear ratio given this info?  

 

Looking at your last photo (I also rotated it 180 degrees) I can clearly make out a stamped number 3 .. and what "maybe" a second character of 5 and third character of 4.  So perhaps it is a 3.54 3rd Member.  If that is accurate ... you keep what you have and look at other options to increase speed for the highways.

 

To confirm your ratio look at a second location where it also may be stamped on the outside ... see my second photo below .. it is a flat area right next to the fill plug that Plymouthy Adams mentioned in post #10.

 

The last location where it will be is on the main gear inside the 3rd Member itself.  (Follow JB's recommendation before you remove a 3rd member)

 

My third photo shows where is it stamped on the main gear ... highlighted in red.

Edited August 17, 2016 by Phil363

[Desotodav]

Do you have a flat section on the diff just below the filler plug Chris?... as this is where the ratio was stamped on the later model trucks. I couldn't quite see that area from your photos. Here's a pic of the red diff in one of my trucks showing the area of that ratio stamping...

[Young Ed]

Looks like you might be good to go already. I think your next step now is a tach and see what your RPMs really are at speed.

[Plymouthy Adams]

 while the axle is a banjo type...the legendary 8 3/4 did not come into play till 1957..your casting number is much lower than the identified carriers castings for the 8 3/4....HOWEVER..... as it is a 3.54 you are lucky that someone prior to you has found the holy grail of early carriers from a larger Chrysler car and retrofitted already.  This is the best ratio you going to get for that axle housing...

[HotRodTractor]

Running some quick Google calculator that should put you in the range of 2100 rpms at 50mph. (I estimated tire size - I'm not sure what size you are running)

 

I agree - find a cheap digital tach that you can just wrap a wire around a spark plug wire and get a good RPM reading at a known speed to make sure everything is in line.

[Reg Evans]

Your speedo is probably not correct with the 3.54 ratio.  Bring along a gps to check your actual speed.  I have a 3.0 ratio in my 52 Pilot House and my speedo is approx 25% off.

[wayfarer]

...for those w/o the fancy gps stuff, the hiway mile-post markers are fairly accurate...

[chrismfischer]

Thanks everybody! I will check again near oil plug for a ratio, and do the tire/driveshaft spin test too and update. Need to get a tach to see if my ears are deceiving me on the high rpm sense I'm getting at 40 mph + (gps speed).....speedo reads about 29 at 40 mph true speed btw.

[Plymouthy Adams]

these things do sound busy...not anywhere near the refinement in isolation of mechanics let alone the body sound deadeners of the modern car..

[chrismfischer]

Ok, I'm back.  I found no additional casting numbers or stamped numbers on my 3rd member (no jokes here, please).

Jacked up rear so both tires off the ground. Spun one tire, the other spun in opposite direction.  Then I chocked one tire, and spun the crankshaft 10 revolutions.  The Free tire turned approximately 5.65 revolutions.  Did it twice, same result.

 

Does that mean I have a differential gear ratio of 10/5.65 or 1.77 : 1   ?   That seems like an outrageously low ratio.  If that's true, it implies that if my engine rpms are truly high at cruising speed, my tranny must be geared with a high-high ratio.

 

Next I will buy a tach and test out engine rpm vs speed.  Unfortunately, I'm waiting to get my steering gear box back, so I cant test it until I get the steering box reinstalled in the next couple weeks.  I'll update once Ive done that.

 

Question to ponder:  so what is a max cruising rpm on a flathead I6, without fear of longterm engine damage?