Diff, Tires and RPMs Again

[55 Fargo]

Did a speedo test with my 47 Chrysler, equipped with 6 cyl flattie, 3 spd standard trans, dry clutch, either a 4.30 or 4.10 diff, and very short P205 70R 15 tires. The speedo was registering 65 mph while my chase vehicle indicated I was doing only 50 to 52 MPH.

Wow is this speedo out, the fact is the stock speedo, had a 3.54 diff driving the wheels. Now with a 4.30 and tires that only have a diameter of 25.5 to 26 inches, this is really throwing everything out.

The stock Chrysler engine with 114 hp, the fluid drive and a 3.54 diff, pulls away very slow from a dead stop, I know as I have driven one very recently,and in my opinion are quite slow off the line. But once rolling cruise very nice at 60 to 65 mph.

I was thinking, my engine is only a 100 hp, with a dry clutch, there is no fluid coupler as a fluid drive provides, so going with taller gears may make my get up and go performance with a dry clutch and 3 spd trans suffer. This car weighs probably 700 pounds more than a P15 Club Coupe, so it will need lower gearing for off the line performance.

I was noticing it was a little slow after shifting into 3rd gear until I hit about 35 mph, then it would pick up speed a lot better.

So maybe , with the present engine, dry clutch, I should keep a lower rear end, for now. Does this make sense, or am I off on this one.

Having these small tires, gets the rpm really wound up at 50 mph, plan to get P225 75 15 tires, this would give it about 2 to 3 inches taller tires, which would give me a higher speed at a lower RPM.

[De Soto Frank]

Fred,

Here's a short (hopefully!) answer: Europeans have been getting the most out of tiny, wimpy engines for over 80 years; they've done it by using "low final drive ratios" (stump-pullers) and multi-speed trannies (four or five-speed), with the top gear being an overdrive, OR, they use a moderate ratio final drive, with top-gear being direct, and the next-to-top gear being an intermediate cruising gear.

This was the only way that Hondas, and VW's could get up to 70 MPH with their modest engines (up until the imports started their own HP-race in the '80s).

Americans car-buyers made it very clear before 1930 that they didn't want to shift more than 3 speeds in their cars. Chrysler tried to market a four-speed tranny in their high-end cars in 1930-31; they referred to it as having a "double high-gear"; you would cruise in 3rd in the cities/suburbs, and when on long, rural turnpikes, put it up into 4th for easy high-speed cruising. Few people wanted them. The feature was discontinued by 1932.

Overdrive uses the same principle - flexible gearing to match load to peak engine power-band, but it "fooled" the 3-speed ninnys, because it operated "automatically" (almost).

Now to your case: given that you have a fairly big, heavy car, with a modest engine, if you don't want to go the OD route, look around for a 4.1, 3.9, or 3.7 hog'shead; you'll get better driveability yet still be able to climb hills. Forget about drag racing !

If you are planning on adding OD, keep the 4.3 or 4.1 (whichever you have); you'll end-up with the best of both worlds: stump-pulling power when you need it, and highway speed when that is appropriate. Since most OD's give a 27 to 30 % reduction in engine revs, final drivers lower than 3.9 get too tall for the wheezy flat-head (3.9 x .70= 2.73 effective final drive ratio - that's auto-tranny territory!). Guys running OD with GR's lower than 3.9 find their engines lugging in OD, unless they're on flat highway, and above 65 MPH. Lugging is very hard on the engine, especially main bearings.

I finally found the ratio stamp on my '41 De Soto the other week, when I was under the back end replaceing shocks. From the pass. side of the car, look at the hogs-head, where the bolts secure it to the axle housing. Look between the bottom-right bolt (about 7 o'clock) and the next bolt up (8:30 o'clock), between those two bolts, a little forward on the "snout", should be a flat area on the casting - almost 2" square: scrape the crud off that, and hose it down with some carb or brake-cleaner: that's where the ratio is stamped.

If your car has a 4.1 or 4.3, you can cruise at 55 - 60, it'll just sound really busy. Use the tallest tires you can get.

Let us know what you find...

Frank

(PS: only the Fluid Drive semi-auto sixes had 3.54 rears; as with auto-trannies, the "slippage" of the fluid-coupling / torque converter allows the engine to spool-up a bit into its peak power-band to get the vehicle moving / on steep pulls. My '60 Windsor, with its 300 HP big-block and T-flite has 2.93 gears; if you took an identical car and changed only the tranny to a 3-speed stick, the T-flite car would beat it every time; stick & 2.93 would be a dog, unless it were a 300F with the ultra-rare Pont-a-Mousson 4-speed stick!)

[55 Fargo]

Thank you very much for the dtailed answer Frank, as I thought myself, the 3.54 with my present set-up , could be a little tough gettin going, buckin wind and hills.

Hopefully this is clear in my mind now,and I won't be buggin yall on this topic anymore.

I know I can probably cruise at 55 mph with the presnet set-up, could probably cruise faster, but this engine has miles on her. Greg G. was cruisin at 60 to 65 mph, he has 4.10 gears in his car.

As you have mentioned the Chrysler is a fairly heavy car compared to a P 15, so going to gears as tall as 3.54, might not be the best idea, but the OD trans is a good idea, something to consider in the future. I have been asking around here about an OD trans, nobody seems to have an idea where to get one, or if they have seen one. It is possible, that in Canada, they did not have many OD transmissins as they did in the USA.................Fred

[Plymouthy Adams]

I have a moderately heavy 54 Plymouth Savoy, it is a 230CI 110 HP engine. Other values are automatic transmission and 3.54 rear ratio. Given...! I do have a torque converter verse fluid drive coupling. This car will burn a bit of rubber in first and accelerates quite nicely and shifts smoothly at various speeds dependent on the postion of the throttle in regard to the kickddown pressure. If floored and held at full throttle it usually shifts into high at 53-55 MPH. I don't consider this car a slug by any means..the ability to float along at 70 MPH is effortless..the engine seems very content at this RPM..at 55MPH you want to open the door and see if the engine is still running...The 3.54 makes a real difference as compared to my previous owned '49 with the 218 and standard three speed and 3.9 gears. I am also running stock bias tires of the correct size as stated by the manual for this year car. I will admit I have not gone out and one any extended run to calculate mileage, MPG or odometer accuracy. Maybe later for that info...Louman Earle has driven this car and can verify the above data...to say I was pleasantly surprised is a fact...I expected a dog, though it is not a greyhound...it holds it own very well indeed...

I do not have any substained hills to pull but do have a few elevated roadways and it pulls these well. The times our cars were built was in days of few super highways and most roads were unimproved per say in comparison to todays standards. The ratio's of old could easily be replaced with higher numbers and still do well. Items such as the taller gears and higher tires are prevelant today as manufacturers strive for great fuel mileage. Lots of the truck out there today don't seem as powerful as their earlier counterparts but will walk the dog on them on the top end with better MPG's to boot due in most to OD trannies and these taller tires...

There will be trade offs...are you looking for a cruiser with good mileage and long engine life or as some folks terrain in which they live may dictate these babies be stuck with lower rear gears and busy engines. Larger tires do take a bit more to spin up per say but the overall economy by fewer RPM of both the engine and tires result in longer life of both items.

[Brendan D25]

Are the D24 differentials and the P15s the same width and are the crown and pinion set (hogs head) interchangeable? Reading this thread I remembered where there might be a D24 rearend. Brendan.