Technical Hi, I'm Roy, I own a 1932 Plymouth PA 3 Window

Hi Guys, looking for overdrive transmission options for my 32 plymouth 3 window, I'm running a 340 plymouth, 4barrel, 904A torqueflite, 8 3/4 rear (not sure on gear ratio) but I get terrible gas mileage. I like to drive car around state for shows but maybe getting 10 miles per gallon highway driving, any suggestions? Thanks

 

It needs a tune up, not an overdrive. Jack E/NJ

 

It is a Hot Rod. I cannot tell you what kind of mileage they get. I do not care, I just drive them.

 

Oldtimer , i would start with a tune up as mentioned and than your'e really going to want to know what gears you have in the rear. You can jack up the car and count the rotations of the tire and driveshaft ( lots of info on the internet as to how to do this). There are calculators on the internet ( Wallaceracing.com) that will help you determine rpm changes using new gear sets and/or tire height changes, very helpful. Torque converter slippage may come into play if it's not a good one. A 340 is an awesome engine as i'm sure you already know, i'll probably start by looking at the carb first. If all that checks out ok take a look at Gear Vendors overdriver units. They bolt onto the back of your trans and give you .78 overdrive, a little pricy at 3,000 but could be a problem solver. You shouldn't have to change your trans tunnel which is a huge plus but would have to shorten your driveshaft, not that big of a deal.

 

Hi Roy
What rpm is the engine turning at highway speeds?
It would be helpful to know what gears you already have as Your easiest option would be taller gears in your rear axle. Its Easy to swap out the whole carrier for one with more highway friendly gears with an 8 3/4 like yours.
As far as overdrive go you could potentially use one from a 2wd 318 Ram or Dakota from the mid 90's I believe they are called 46RE.
I'm sure there are some mopar guys on here that would know more about this swap than I. What about it guys?

Steve

 

The cheapest and easiest way to compensate for low gears and increased your gas mileage is taller rear tires

 

Thanks for feed back guys will look into it.

 

Welcome and as mentioned above start with getting the Motor giving max performance. Just because it starts okay and moves down the road don't mean it's Tune Up is giving 100%. Those small blocks generally got around 14 mpg in stock form. You haven't added enough curb weight to get off that mark. Find someone that really knows there stuff and get that motor running at it's peak, then tackle gearing. As mentioned Tires are generally the least $$$ spent for better mileage.

 

Back in the day, my stock '70 340 Duster with 3:23 rear got between 10 around town to 14-15 on the road.
That body was a little more aerodynamic but similar weight
Of course I was 17-18 years old at the time, so.......!!!!

 

I had a 340 in my model A and in my Dart. Don’t know what mileage I was getting but sure had some fun getting there. 10 isn’t horrible, I’m sure you could squeeze out some more. Is it running well? Good acceleration?

 

Check your vacuum advance.

 

How about a photo of your car? Those are nice cars but you don't see as many as you do Fords and Chevys.

 

One other thing you can try is just using Non-ethanol gas, we have seen small increases in MPG on some Hot Rod and muscle cars. As others have said, a proper engine tune is vital. If you can share what your engine rpm is at 60 & 70 mph, that would be good to know.
Is your speedometer correct? Check it against a GPS.
In our many years of experience at our Hot Rod Shop, we have not seen allot of improvement of overdrives with our old engines and carbs. Today's junk ethanol gas is a major reason. We typically see 8-10 MPG in town driving and 10-14 MPG on highway depending on the car and driver.
Hope this is of some help.

 

340's get great gas mileage. Dad's 69 340 4spd Dart Swinger got 24 mpg with all 6 of us in the car and loaded to the gills on a long camping trip. 3.23 posi gears. We were so loaded the tent was tied to the front inner fender panel under the hood. I have several 340's, all of them get good mileage.

Dave

 

I did’t see anyone mention it by any overdrive kit will cost you at least 5k and a new driveshaft. I would look into the gearing and tires.

 

If the motor is running right you must have a large gear in that thing.mylast 340 powered dodge got about 20 on the highway 3.23 gear.

 

Dave, most did I'm sure but I recall one in particular that did not, it belonged to the older brother of a high school classmate of mine.
The car was a near new, 1970 AAR Cuda, 340, 6 pack, auto, seems that combination with the day two change to a nasty Sig Erson cam made this one tempermental car but when it came on the pipe that thing was an absolute animal, and I'm betting it got no better than five miles to the gallon.

 

I know....picky. A PA Plymouth is 1931. PB and PC Plymouth are 1932.

 

When people ask me what kind of milage I get with my roadster, I reply: "I don't care! If I was concerned, I would be driving a "Prius"...

 

PA's carried over into 32. I have seen a couple. Also 1930 model U carried over into 1931.
I have a 1931 PA, 56 Chrysler Hemi, 727, 8 3/4 with 3:90 gears. Mileage unknown. I just drive it.
Check your tune up, carb size and adjustment.
Taller tires if they fit is cheap.
Check your gear ratio. May want to find another 3rd member with lower numbers.
Good luck! Post pictures

 

If overdrive is in future use a 90-95 dodge 318 46rh. No need for fancy electrical contraption. A easy toggle switch for od. And you can do away with lock up converter with a few mods to transmission so you can choose what torque converter you want. Fairly simple and straight forward.