Change rear axle 3.9 to 3.73?

[central52]

At about 50 mph, my 47 Plymouth screams for a lower gear ratio. Aside from trying to install an overdrive, can the gears in the differential be changed without too much fuss? And is it expensive? Ed P.

[55 Fargo]

At about 50 mph, my 47 Plymouth screams for a lower gear ratio. Aside from trying to install an overdrive, can the gears in the differential be changed without too much fuss? And is it expensive? Ed P.

http://www.4lo.com/4LoCalc.htm

Here is a calculator to figure out what you may gain, not a lot by going to 3.73, going to 3.54 would make a difference.

Going with taller tires would help too.

With 205 75 15 tires, and a 3.9 diff, at 50 mph you would have 2400 rpms, with a 3.73 diff it would be 2300 rpm, not a whole lot of savings.

Your engine and set-up should be able to cruise all day at 55 mph, maybe evn up to 60 if the engine and all are in fine form. If you want to cruise at 65 to 70 mph, your going to neeed an Overdrive...........PS the gears are easy to cahnge, as long as the pumpkin has the same spline size for the axles as what is in your car now. The splines are 10 on the axles, and sewitched to 16 in the 50s sometime. I have personally done this swap with 4.3 gears to a 3.73 gears. Are you sure you have 3.9 gears, it will be stamped on the hoghead on the diff itself......

[claybill]

are not all p-15's 3.9 rear-end ratio ? what would be the exceptions?

i read somewhere that the 7pass/limo was 4.3 that makes sense to haul the extra weight. how about convertibles..were they 4.3 as well..? why?

interesting why motor co.'s decided n one ratio as oposed to another...

any comments.?

plymouth advertised that you could drive using 2 gears instead of 3 except on a rare occasion....that could be one reason..and speed limits were 50 mph max. etc etc...

claybill

[40phil41]

I changed the gears in my '41 Dodge from 4.1 to 3.73 (from a mid-50's Dodge). However, it wasn't a direct swap since the axle splines had a different number of splines and I didn't take the axles when I pulled the 3.73. This may not be an issue with your P15; just something to be aware of. The 3.73 did make a significant difference in driveability at highway speeds compared to the 4.1.

Phil

[55 Fargo]

are not all p-15's 3.9 rear-end ratio ? what would be the exceptions?

i read somewhere that the 7pass/limo was 4.3 that makes sense to haul the extra weight. how about convertibles..were they 4.3 as well..? why?

interesting why motor co.'s decided n one ratio as oposed to another...

any comments.?

plymouth advertised that you could drive using 2 gears instead of 3 except on a rare occasion....that could be one reason..and speed limits were 50 mph max. etc etc...

claybill

I think for the most part the 3.9 was the diff of choice, in Mountain regions the 4.10 was an option.

My 47 Chrysler club coupe had a 4.3 diff,supposed to be 3.54 or 3.90,these cars are old, lots of things can happen in 60 years, like someone swapping diffs etc....

[greg g]

I run 4.11 in my 46 p15. I am running 215 75 15 tires according to the calculator is it running 3100 rpm at 62 mph. HP rating is at 3600 anything under 85% of that is the engies comfort zone. Remember a lot of the noise you are hearing is fan noise and lifter noise. Noises we aren't used to compared to our new cars going down the road with the windows up ac on crusing at 70 at 1800 rpm's with no fan noise, and nice quiet radial tires.

[Reg Evans]

I have a NOS set of ring and pinion gears.....3.54 ratio. Part #1124304.

Handwritten on the box says 42-52 Plymouth, 41-52 DeSoto,and 53-54 Dodge D44-D48w/torque converter and D50-D53 w/power flite

Make me a good offer at regoevans@sbcglobal.net before it goes on eBay.

I paid $180 for it in Feb. 2005.

I had another set that looked just like this and it went into my 49 Dodge to replace the 4.11

Grandpa needs a new pair of shoes:o

[Norm's Coupe]

Bill,

Everything I've read on the P15's say the rearends were either 4.1 or 3.9. I have the 3.9 in my coupe. Was there when I bought the car.

[krodaddy]

Will a 3.73 ratio work with a overdrive transmission?

[Plymouthy Adams]

P17,19,22,24,25 got 3.73 except for hilly country whereas 4.1 (short wheelbase except for 53/54 when all cars were same chassis)

all HY-Drive were also 3.73

else..3.9 or 4.1 depending on terrain of areas car was shipped to..

[Young Ed]

OD cars came with 4.11. Dad runs both of his with a 3.9. 3.73 might be a little too high for the OD.

[wayfarer]

Here is another 'calculator', pretty user friendly, lots of input options:

http://ctny.audiworld.com/mark/s4/gears/gear_calc.html

.

[Plymouthy Adams]

actually the OD in my opinion will be well driven by the flathead as its max torque is just at 1200 RPM...I know a number of these cars running OD with no apparant problems..remember..the highways of today are a lot better than in the years these cars were being produced..the grades are longer BUT less steep...

[wayfarerstranger]

i couldn`t get the chart top work ... on a 49 ply w. wagon rear 4.1 with L78`s on the rear what would that equal to ? ,, and if i go to a 3.73 what would it do for me ??

[Powerhouse]

my 3.9 rear is sluggish on hills...and almost useless with overdrive on...my non-electric OD makes for a very "Plan Ahead" driving experience. If I know hills are coming...and they always seem to be around here...better not have it in OD...or the snails will pass by.

I wonder if I should get a 4.11 rear for my 39 plymouth???

[Don Coatney]

OD cars came with 4.11. Dad runs both of his with a 3.9. 3.73 might be a little too high for the OD.

I have 3.55/1 rear gears with a 30% overdrive and my flathead 6 Desoto powered Plymouth pulls all but the steepest of hills with no problems.

[Young Ed]

Don you have a little more Hp then stock too.

[55 Fargo]

i couldn`t get the chart top work ... on a 49 ply w. wagon rear 4.1 with L78`s on the rear what would that equal to ? ,, and if i go to a 3.73 what would it do for me ??

Chance here are the calculations,

L78 15 tires + P23575R 15= 28.9 inches

4.10 Differential @ 50 Mph =2383 RPM @ 60 MPH = 2860 RPM

3.73 Differential @ 50 MPH= 2168 RPM @60 MPH=2601 RPM

Hope this helps you out...............Fred

[Norm's Coupe]

..the highways of today are a lot better than in the years these cars were being produced..the grades are longer BUT less steep...

Tim,

Evidently, you have not driven the freeway to Branson, MO (hwy 65) from I-44 in Springfield, MO. There are several long grades, straight up. I think they forgot what the word gradual grade meant when they built that road. On some of those grades you have to be doing about 90 or 95 going down one hill before you immediately go up the next one. Even then if you don't really give the car the gas when starting up the next one, you'll be slowing down 45 or 50 MPH when you hit the top of the second one. That was the way I rode those hills a couple of years ago. Otherwise I would have been like everyone else. Staining to get to the top, even in cars that had big V8's under the hood were struggling. BobT knows what I'm talking about, I'm sure.

That road is much worse as far as hills go than the ones through Pennsylvania, Virginia, W. Virginia and eastern Tennessee.

[wayfarerstranger]

YES THAT HELPS ! looks like a good difference ..

[RobertKB]

If anyone is interested, I have a good looking 3.73 for sale from a '54 Canadian Dodge which has identical mechanics to Plymouth except the longer engine. Shipping could be UPS, Fedex, DHL, or possiby post office. PM or email if interested.

[central52]

So, would there be a significant difference in performance and gas mileage if I go from a 3.9 gear to a 3.54 when I travel the parkway at 50 mph? Since I only drive the car once a week for about 20 miles, (parkway & street), would I gain a lot by switching? I'm running on 600.16 radial tires.

You know the old saying, "if it ain't broke, don't fix it". Ed P.

[Young Ed]

If anyone is interested, I have a good looking 3.73 for sale from a '54 Canadian Dodge which has identical mechanics to Plymouth except the longer engine. Shipping could be UPS, Fedex, DHL, or possiby post office. PM or email if interested.

I'm interested! What are you asking?

[james curl]

I have a 3.55:1 rear end in my 48 plymouth with 218 engine with a few modifications and on the trip through Colorado and Utah going to Hot August Nights in Reno and on to Bonneville the only time we had to use second gear to climb a hill was at Monarch Pass at 11,312 above sea level. All of the hills around Reno and Tahoe we pulled in third or high gear.

[wayfarerstranger]

high gear ??