Overdrive VS. Differential...Another Discussion

[ccudahy]

I have been observing posts about transmission upgrades and rear end upgrades, to achieve the same thing lower RPM's for the engine and higher highways speeds. So, which is the better upgrade? Which has the biggest bang for the dollar? Which would you personal do or have done and why?

Edited February 14, 2021 by ccudahy

[Plymouthy Adams]

that depends greatly on the terrain you drive as to whether a rear end change would do what you need.  Rear ratio change would affect all gearing forward and reverse.  Whereas the OD would be the single gear reducing the rpm at speed and you have all the power and torque of the ratio built into the car as designed.   There will be a sacrifice of power and slow to attain speed in the lower gears with a rear axle change.  Hilly county and you could find yourself not liking the difference.  But economically, a rear gear cost is much lower initial cost and will also give you modern braking components.  You could be faced with driveshaft costs also that can elevate that initial cost and get you closer toward the cost of an OD.  

[greg g]

The simplest and most cost effective change is taller read larger overall diameter tire.  A 25 inch tire will give you about 80 tire Revelations per mile,go to 27 or 28 and you are nearer 70 revs.  There wasn't a lot of choice of rear end ratios when these cars were built 4.11 to1 for hilly country and 3.89 to 1 for flat Landers. Going to a lower numeric ratio like 3.55 or so is as noted ok if you are cruising around, not in stop and go and lightly loaded.  A car with a 411 rear end and stock size tires will go down the road at around 3300 rpm at around 60 mph. R 10 Overdrive will drop this to about 2400/2500 rpm..  so you have in ton traffic keeping and relaxed cruising.  Over all cost in fuel and wear and tear on an occasional use car???? Who knows where the break even point maybe.  Overdrives are neat work well but are getting very expensive and changing the break even point if that's your mesuring stick.

Edited February 14, 2021 by greg g

[keithb7]

I’ve considered this topic too. I live in a mountainous area. My old sedan has a 4.11 crown and pinion set. It makes sense as the sedan could be loaded up with 4 people and luggage. 4:11 would handle that pretty well. 
 

My car cruises comfortably at 50-55 mph. Above that she’s zinging pretty high for my comfort level. Yet she pulls hi-way hills easily at 50 as well,  and can hold there.  I’m sure lower rear end gears would kill the engine rpm on the same hi-way hills. 
 

Most of my driving consists of cruising in town. Even when I do get on the hi-way once in a while, over 55 mph is out of my comfort range in a 1938 car. 
 

This winter I’m boring up my engine size from 228 ci to 237 ci. This should net me good hp and torque gains for the steep hill home. It won’t lower high speed engine rpm on the hi-way any though. Yet if 3:95 or so rear end was available I’d consider that. I’d bet my new engine could pull it handily.

Edited February 14, 2021 by keithb7

[ccudahy]

Thanks for all the responses so far.

 

I have what I believe to be a stock '47 Plymouth Club Coupe, 218, standard 3-speed. I don't know what rear end ratio yet?

 

I honestly don't know which way I would decide. It sounds like you have to give something up to get something else and its a wash in the end, and better off leaving it alone. I live in a relatively flat area, couple of big hills here and there. Also a good network of 55 mph roads. Really just don't want to get run over on the highways and do it that the engine isn't screaming. I really don't know how the car acts, have not had a lot of windshield time, since buying it trying to make it roadworthy.

[plyroadking]

In order to achieve the final drive ratio an overdrive provides you'd have make a big drop in a rear end ratio. One ratio calculator i like is http://www.public.asu.edu/~grover/willys/speed.html#:~:text=Engine RPM divided by total,gives vehicle's speed of travel.

 

It allows you to put in an overdrive ratio and play around, R10s are a 0.7 I swapped out my 3.9 ratio 3rd member to one from a 50s dodge that's 3.73. That only gave me a 3mph speed increase. In my opinion adding an overdrive is the less intrusive and easily reversible route to go. I've logged around 80k miles with my '40 that has an overdrive in it. I used to live in the Sierra Nevadas and used 2 gear with overdrive many times climbing the hills, when I lived in the bay area it was really nice to be able to keep up with everyone else doing 80mph in the 55 zones. Now that I'm in Iowa, I'm considering swapping in my 3.54 ratio 3rd member for crossing the plains. 

Overdrives can be expensive if you just want to pay for one to show up on your doorstep. I've collected and sold many of them, I usually had to buy the entire car just for the transmission. They're very well built and just about every used I've torn down was in a condition that I could have just changed the fluid and slapped in the car.

[Loren]

Well my 1952 Suburban was built  before the Interstate highway system so its preferred speed is 55-60 since it has a 3.73 rear end.

An Overdrive with its 30% high gear gets you to modern speeds painlessly. A 1940 (one year only) gear set lowers 2nd gear for better acceleration and to utilize the Overdrive more often in 2nd gear.

I have a 3.54 pumpkin (from a 1956) for the stock rear end if I can get more horse power to pull it.

To get the extra horse power I've built a 265 Chrysler which is in waiting for it.

More power and more speed indicates an upgrade to better brakes and that is addressed with 12 inch Chrysler Windsor brakes.

Everything is period Mopar and everything is available if you do some shopping for a decent price.

Overdrive prices I believe have peaked. The days of cheap Ford and Chevy Overdrives and expensive Plymouths are gone. They're all expensive but bargains are now appearing in the Mopar realm. That's my 2 cents and your mileage may vary.

[rallyace]

I went the overdrive route.  It took me two years to find a good OD unit at a fair price and near enough so I could see it and check it out before consummating the sale.  In my case it was a very good unit and only needed to have the hypoid oil flushed out and a couple of seals needed replacement.  A stock driveshaft will work with an OD transmission swap.  I did have a new driveshaft with standard Spicer universal joints built by a local truck driveline shop.

[greg g]

The fellow who did the machine work on my 56 230 engine told me that 80 to 85 % of peak HP rpm was a sustainable cruising rpm, assuming a realitively good condition engine. So pull up a hearing calculator on line and plug in your numbers. If your calculation is in the neighborhood of 3100 to 3300 rpm at 60 give or take mph you are in the slot so to speak.  

 

Now if you compare that to a new car with the windows up and the radio on with the ad blowing and the tach reading 2400 or less to the sound of a flathead six with the windows down rolling along at 3200 yipes that sure sounds busy... How much of it is fan noise, tire noise and wind noise??

[Booger]

running  at 60 mph and 3 grand rpm youre running the dog snot out of that little machine. No bueno, senior. just looking for longevity. get on that treadmill run your heartrate to 200 for an hour. thats not where I wanna be

[greg g]

Before I installed my OD,  we made many trips of 500 or more miles.  This was on a fresh 230 rebuild.  With the 4.11 rears with 225 75 15 tires, indicated 65 on the speedo was 62 mph on the GPS.  This calculated to 3280 rpm. From home near Syracuse we went to Detroit 2x, Charlotte, NC, VIRGINIA, Maryland, northeast Vermont, Mass, the Philadelphia area, Gettysburg,  usually one 500 mile or longer trip per driving season.  Tossed a fan belt near Gettysburg, and had a split fuel filter in Michigan. 

[48ply1stcar]

Last Spring I bought a 2000 Dakota 3:55 rear axle in a junk yard for $150.  A shop in Anoka Minnesota, disassembled it, I had it sand blasted and painted.  They reassembled it with all new brake parts that I bought from rock auto.  I also bought the mounts on-line.  They built a driveshaft.  All brake parts a new, I also installed Scarebird front disc brakes.  These are best things I have done to my car in the 52 years that I have owned it.

 

[MackTheFinger]

1 hour ago, 48ply1stcar said:

Last Spring I bought a 2000 Dakota 3:55 rear axle in a junk yard for $150.  A shop in Anoka Minnesota, disassembled it, I had it sand blasted and painted.  They reassembled it with all new brake parts that I bought from rock auto.  I also bought the mounts on-line.  They built a driveshaft.  All brake parts a new, I also installed Scarebird front disc brakes.  These are best things I have done to my car in the 52 years that I have owned it.

 

That's good info. 

[Frank Elder]

Got rid of my 3 speed fluid drive and replaced it with a standard standard so to speak......?.....better response and yes better gas mileage. If I was ever to get a Borg Warner 4 speed OD it would be a  non fluid drive variety, what a slug no salt needed to slow it down.

Edited February 17, 2021 by Frank Elder

[ccudahy]

Thank you @Frank Elder I was just contemplating maybe a fluid drive would be better. That clears that up.

 

I know my wife was more warm to the idea of shifting (using the clutch between gears) with a "4-speed" semi-automatics found in the early '50s. She already said she doesn't even want to learn or try driving the '47.

Edited February 17, 2021 by ccudahy

[Loren]

I have never driven a Fluid Drive car and have always found the concept interesting and the ultimate version the HyDrive even more so.

My Great-Great Aunt Clara had a HyDrive Plymouth and still talked about it years and several automatic Mopars later.

The idea of using the engine oil as a medium to cool the torque converter isn't real appealing though. The torque multiplication function really builds up the heat. Fluid Drive without torque multiplication didn't need extra cooling. If ever you've seen a centrifugal oil filter you'd think twice about running engine oil in anything that spins. You have to chisel the sludge out of them. It's hard as a rock!

I have an MB GLK 250 with their 7 speed automatic. In that car you have to change the fluid and filter at 70,000 miles. If you don't the transmission slips in high gear. The dealer never drains the Torque Converter even though its expected (neither do the independent garages) so you never get to 70,000 again without the transmission slipping. Because of that I do my own transmission service and the fact they charge $800 for it!

A Borg Warner 4 speed (T10?) could be had with an Overdrive adaptor. They are super rare and I've only ever seen one adaptor on eBay. The adaptor was made by a friend of my Dad's Tom Beatty. I am sure a T5 might be a better alternative (if you could get the right gear ratios). Most folks source them from S10 pick ups and they had awful gear ratios even in the automatics. One place I worked had an S10 with an automatic during the 55 mph speed limit. Trouble was the truck could not be driven at 55...50 or 60 were your only choices. I bought a Model A with a T5. The ratios are so bad it's coming out and a 39 V8 transmission is going in.

I've never seen the advantage of changing a Plymouth rear end to something else.

The Mopar 8 1/4 has ratios 3.54 to 4.3 is well designed and well built. It uses a 3rd member design which makes it real easy to change ratios. What's not to like?

The brakes can be uprated from 10 inch drums to 11 or even 12 inch and you don't have to change anything else. All the hydraulics are the same. Seems like a lot of work to put something else in a Plymouth for so little return on investment.

[47 dodge 1.5 ton]

Why not use the 8 3/4” mopar rear? I always wondered why many on here go 8 1/4” or 9 1/4”? 

[48ply1stcar]

2 minutes ago, 47 dodge 1.5 ton said:

Why not use the 8 3/4” mopar rear? I always wondered why many on here go 8 1/4” or 9 1/4”? 

I used a 8.25 from a 2000 Dakota because the dimensions make it a easy bolt-in replacement, even though I didn't do the work 

[48ply1stcar]

At one time I toyed with the idea of using the Wilcap adapter and going with a Chevy automatic transmission.  I guess I never considered a 70-year-old overdrive transmission, just keeping is simple and not concerned with using original parts.

 http://www.wilcap.com/chryslerfh.html?230350BOPAT

 

[Plymouthy Adams]

1 hour ago, 47 dodge 1.5 ton said:

Why not use the 8 3/4” mopar rear? I always wondered why many on here go 8 1/4” or 9 1/4”? 

 

the source for these is drying up and the cost surrounding one if found makes them a bit out of the range for anything less than the strength you need in all out drag....for the street and most warmed over engines...the 8 1/4 will suffice.....they were the 8 3/4 replacement and have been around for many years and finding them is almost as common as shells on a beach thus the go-to today

[HotRodTractor]

Don't forget about the possibility of using a Ford 8.8 axle - the width can be found that is correct - and they are plentiful, cheap, and strong. I'm a proponent of swapping both the axle and transmission - mostly because when going to something like a T5 you lose the emergency brake, which a new rear axle can give you, plus upgraded brakes, better ratio, locker, etc... The Ford 8.8 I have in the shop right now cost me $100 - it came out of a Mercury Mountaineer and included the entire rear half of the frame and the driveshaft. It has 3.73 gears, limited slip, and disc brakes. 

My experience with Fluiddrives is limited - in fact I have only driven one and it is in a tractor. I didn't find it sluggish at all. In fact I was surprised at how responsive, yet smooth it is. It is very easy to throttle up and power right through the brakes. Granted - I'm dealing with a lot more gear reduction in a tractor, so that might be a contributing factor. 

I have one of those Wilcap adapters - they are nice well made products, but I'm using it to swap a flathead Chrysler into a Model A Ford. (Also mine is for a manual transmission). Its also nice they allow you to setup a starter on either side of the engine.... but be careful as many bellhousings will not allow either side. I had to find a "dual pattern" bellhousing out of a 1982 F-body to make everything work out correctly and get the starter onto the driver's side. I'm sure similar issues exist on the auto trans side of things. Just something to be aware of if you want to keep the starter on the driver's side. 

[Bob Riding]

On 2/14/2021 at 8:30 AM, Loren said:

Well my 1952 Suburban was built  before the Interstate highway system so its preferred speed is 55-60 since it has a 3.73 rear end.

An Overdrive with its 30% high gear gets you to modern speeds painlessly. A 1940 (one year only) gear set lowers 2nd gear for better acceleration and to utilize the Overdrive more often in 2nd gear.

I have a 3.54 pumpkin (from a 1956) for the stock rear end if I can get more horse power to pull it.

To get the extra horse power I've built a 265 Chrysler which is in waiting for it.

More power and more speed indicates an upgrade to better brakes and that is addressed with 12 inch Chrysler Windsor brakes.

Everything is period Mopar and everything is available if you do some shopping for a decent price.

Overdrive prices I believe have peaked. The days of cheap Ford and Chevy Overdrives and expensive Plymouths are gone. They're all expensive but bargains are now appearing in the Mopar realm. That's my 2 cents and your mileage may vary.

Very interesting about the '40 gearset. I have the stock 'P10 (1940) third member with a George Asche OD that works great in my 1940 Suburban. I can easily spend a significant amount of time in 2nd and 2nd-over around town. In my current '52 Suburban project, I installed a Ford Ranger rear end (for the parking brake setup) so  I have the spare 3:73 pumpkin that came out of the '52. I was wondering if it would be an improvement to transplant it into the '40 - obviously final gear would allow lower rpm cruising on the highway, but I wasn't sure about whether it would be worth the trouble, especially after your comment about the one year only gearset. I thought the the '40 and '41s were pretty much the same under the sheet metal...

[Plymouthy Adams]

do your research...there is also a fast first and second combination...I know of only one person who has capitalized on this build also by adding OD to the combination.....

[Bob Riding]

You ARE my research!

[MackTheFinger]

My two cents, OD would be preferable to rear gear but if you can live with losing some low end go for the differential swap. I thought about doing something to my P-15 but haven't changed anything. It is what it is.