Rear axle measurements

[Joe Flanagan]

When I'm looking for a new rear axle for my 49 Plymouth, what are the measurements I need to know? I have a reference sheet that describes rear suspension width in terms of flange to flange. I'm not sure what they mean by "flange," though. The other way they have it measured is from backing plate to backing plate, which I understand. So if my backing plate to backing plate measurement is 58 inches, I need to find something that's the same width and has the same lug pattern, I assume. Also, when you do locate a replacement rear end, how can you find out what the gear ratio is? Is there any way to tell by looking at the unit itself?

[teardrop puller]

Joe

I used a 78 Dodge rear end. It is Mexican and probably a little different (Dana 44), however the measurments should be the same.

kai

[Plymouthy Adams]

Joe..couple things hre you want to keep in mind...the overall width flange to flange is the mounting surface on the axle faces...the measurement in backing plate to backing plate and the distance between the spring perch centers..

Know ahead what ratio you are looking for and know how to identify the axle in question in this manner...late models have tags on the axle cover bolt and also is on the engine compartment tag of trucks and vans..else it is on the build sheet but decoding is a must.

The track of your car is 56 inches..this is also important. (center of tread one tire to center of tread opposite tire) My Suburban has a 8 1/4 Dakota, I have a 9 1/4 under the Clup Coupe and 8 3/4 under the 41 Bz Cp The Suburban is the only one that will be close to stock and the Dakota axle and wheels fit like a glove...I got the 2:94 ratio and will be using the 4 speed auto here..no biggy as I also have the V6 pushing it. You may opt for the 3:23 or 3:55..think a lot of the 4WD had 3:79 ratios...the early Dakota will also yield matching bolt pattern. The 5th Ave and Diplomats use very high ratio rear gears, double isolated springs. Spongy setup..use your original springs and spring mounts and weld to newly selected axle..when measureing for placement of the mounts...meausure correctly..some pumpkins are offset...unequal axle lengths. centering to the backing plates will work every time...

Changing axles also brings up other probs...in using the Dakota I got smaller brakes..not what I wanted so I changed the backing plates from the 5th Ave..who's rear gear is 7 1/4 2:32 ratio but with the larger 2 1/4 10 inch brakes, they will bolt straight to the 8 1/4 Dakota gear.....take the entire backing plate and brake assy from one to the other.

[Joe Flanagan]

Tim,

That's great information. Thanks very much. I might be making a trip to the scrap yard this weekend but I'm not sure. Still welding in the drivers side floor.

When you say 3:23 or 3:55, what do these numbers mean? This seems like a pretty standard range for updated rear ends. I would like to be able to cruise at 70-75 on the highway without too much strain on the flathead. I assume this ratio will let me do that. Am I right? What is the stock ratio of a 49 Special Deluxe, generally?

It looks like 5th Ave., Dakotas, and Diplomats are all possible donors.

[greg g]

The numbers refer to the difference in input turns to output turns of the rear end. If your original is for argument's sake 3.89 to one, it simply means that the drive shaft must trun 3.89 times to equal one revolution of the axle. So the lower the number, the fewer engine revs (assuming a 1 to 1 high gear) are necessary to turn the the axle one revolution, hence a more relaxed engine rpm at cruising speed. The trade off is that it puts more strain on the engine when starting off from a stop. Remember when our cars were built, the interstate higway system was still a dream and most surface highways were posted for 50 to 55 mph. With starts and stops and going through towns and villages the average speed on a trip was probably in the 45 MPH range, so quick pick up was favored over relaxed highspeed cruising.

With a flathead of around 100 HP and the stock tranny, I wouldn't go numerically lower than 3.23 to 1 and if you need to deal with starts and stops on hills, 3.55 is likely preferable. Either of those would likely give you about 2800 to 3000 engine Rpms at 65 to 70 MPH. this you can vary some by adjusting the diameter of your tires ie taller is the same as a lower numerical axle ratio.

[Joe Flanagan]

So when people write 3:55 they really mean 3.55? It doesn't seem like there would be much difference between 3.89 and 3.55 in terms of performance, but apparently there is.

[Plymouthy Adams]

I'm guilty of the ":" Greg answered that pretty well..and I would also caution against going below 3.23 with stock gearing..actually the 3.55 should you go with stock OD would give a net drive of 2.48

gear ratio when you are figuring rpm per axle rotation is a multiplier of the gear ratio times the final gear ratio..where first gear of 2.90 times the stock 3.9 rear gear would be 11.31 first gear that is why first gear can snatch a 3000 lb car from stop to a quick roll..

third gear is normally 1 to 1 ratio and OD is an underdrive usually in the .68 to .72 range...yeild is about 30% reduction in RPM

[Joe Flanagan]

How the heck do you know all this? This is great. Actually, I always see ratios written with ":", which is why I thought it meant a 3 to 55 ratio, so you can imagine how confused I was.

Also, I do plan to install an overdrive as well. The car has its original transmission, and from what I understand, I'll have to get a different one, since the original is not compatible with an overdrive, right?

[Plymouthy Adams]

Joe..it is a chunk of coin but from all the reports from satisfied customers, the George Asche OD is the only way to go...should last a lifetime with proper service and care over the years...you do not need the high dollar pre war tall second gear..stay stock ratio and all should be within reason with cost.

[Jim Yergin]

Joe,

If you are going with an overdrive I would stay with a higher ratio rear end. That way you will enjoy that ratio in standard drive with quicker acceleration and use the OD to provide gear reduction for higher cruising speeds. I have a 4.1 rear in my car with the OD. Your gear ratio should be stamped in the flange on the differential housing. In other words, I would do one or the other. Either install the OD transmission with your existing rear end or keep the standard transmission and install a lower ratio rear end.

The Mopar OD transmission that George Asche sells will bolt right up to your engine (I guess I know a little bit about that engine) in place of your existing transmission. It should be the same length as the transmission you now have in your car so you can use the same drive shaft you have but you might want to check with George about that. I am sure he will give you the information you need even if you do not buy from him. He is great about sharing information and a pleasure to talk to.

Jim Yergin

[Joe Flanagan]

OK, so I guess I have some decisions to make. If I stay with my stock rear end and get a transmission with overdrive, can I, for example, drive up to New England and cruise comfortably at 70-75 without something blowing up?

I've never driven a three speed on the column. Let's just say I'm skeptical because I've never done it. I do trust you guys who've been around longer, but somehow I just can't see my Plymouth cruising down the New Jersey Turnpike at 75 in third gear. Just can't see it.

And if I just get a new rear end, which seems like the cheaper option, I get the same results? If I can avoid dropping a grand on an overdrive, I'd like to.

[james curl]

Or you can do as Don Coatney did and adapt a T-5 five speed overdrive transmission. Don has a site where he details the entire installation. See Don's profile for a wealth of information.

[Young Ed]

Joe you can certainly do either. I would not do both. The overdrive tranny is the exact same length as the standard 3spd. The extension housing is different to hold the OD parts. If you got a 3.54 or 3.23 you should be able to cruise on the freeway nicely. I have a 3.73 in my coupe and cruise about 60-65. In my truck I have a 3.90 with some really tall 16s and it goes about 60. Mine and dads OD cars with 3.90 rearends will go 65-70. Keep in mind unless you do other changes you will still have a 60 year old brake design that may not be enough at 75mph. Ed

[Joe Flanagan]

What I'm thinking of doing at this point is just getting a different rear end, and staying with the original transmission. Do you see any down side to this? Also, how hard is it to find a new rear axle with the same lug pattern? I really want to keep my original rims. I have all four original hubcaps too.

[greg g]

Well here is the deal, back in the day, when these cars were new and even when they were old and long of tooth, people did drive these cars coast to coast. Now driving down the Jersey Turnpike at 75 in a stock 60 year old car is probably folly on a couple of fronts. Both wear and tear on the long stroke engine, and hurtling along on skinny tires arrested by drum brakes.

This said, I have been driving my 3 speed 4.11 to 1 equiped Bizzy Coupe for 5 years now. I have been from Syracuse to Rhinebeck NY, a round trip of 370+ miles in one day, a trip to Tunckhannock Pa. about 130 miles 0r so each way, and a trip to Killington Vt. about 155 miles each way plus three days af running around while there. On these trips I took the old state routes, Jumping on and off the Interstates as necessary. On the interstates, and on the old state routes I drive between 50 and 68 indicated. Yes the engine sounds busy compared to a modern car loafing along in Od 5th gear with the torque converter locked up probably turning 2200 RPM at 70mph.

Get used to it. Drive the state routes, slow down smell the roses. If ya gotta do some turnpike driving, jump on go 65 and get off as soon as practical. Were it me, I would swap the rear end change the drive shaft, and be done with it. Yes the OD's are neat and you can shift them like a five speed, but with the amount of miles you're gonna drive, the gas saved and the wear and tear saved by either change is likely minimal. If the car is your only transportation, then the change may be worthwhile.

But it yer ride do whatcha wanna do and enjoy.......

[james curl]

You will now have emergancy brakes on the rear drums and the back of the transmission. The yoak on the rearend will have to be changed or you will need a new driveshaft wirh the front u joint from the old driveshaft and a new spicer u joint in the rear to accept the rearend yoak. The driveshaft has to be able to move fore and aft as the rearend moves up and down.

[david lazarus]

I am in tthe process of putting a 3.23 diff into my '38 (out of an XA or XB Ford Falcon) its an easy fit in but also don't forget that your speedo will not be accurate after the diff ratio change.....just something lots of people forget about.