Overdrive or Not This Is The ?

[55 Fargo]

Hey all, not that I can afford to buy a GA OD trans, but would or thought I would like to have one some day.

This what I posted on the Road to Hershey thread.

Hi James, this is interesting, as although I cannot afford to buy an OD right now, I have been on the look out for one.

My 47 Chrysler Club Coupe has a shipping weight of approximately 3500 lbs, my Coupe is minus a fluid drive, and M5 trans, so that probably drops the weight about 100 lbs.

My current mill is a 98 hp 218 long block canuck engine, 3 spd trans, dry clutch 3.73 rear end, tires are P205 75 15.

My heavy car would probably struggle with the lack of HP, perhaps if I drop ina fresh rebuilt 251, with say 125 hp, it would be okay.

I also own a 4.30 diff, George Asche once mentioned to me, that gearing would be great for an OD set-up, but with a 4.30 rear end, how much reduction of RPM can you expect at 70 mph, going witha modern 3.30, might do the same, but of course would be harder to roll off the line, and up hills.

Am I correct with my conclusions, or not?...........Fred

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[55 Fargo]

In my Chrysler service manuals, Chryslers in the early 40s equipped with OD trans had 4.56 rear gears, fluid drive and M5 trans, 3.55, fluid drive and 3 spd trans 3.90, 7 passenger cars no OD trans 4.30 gears, and this is what most likely happened to mmy car, the diff was froma 7 passenger Chrysler if I recall......

[greg g]

Fred ran your numbers through a calculator. Your 205 tires are 27.1 in diameter

So 3.73 to 1 with 1to 1 high gear = 2775 rpm at 60 mph

3.73 to 1 with .73 OD = 2027 rpm at 60

4.30 to 1 with .73 OD = 2335 at 60

Don't think the 400 rpm difference is significant unless you are thiking of a cross country trip, and a pair of 225 15 rears with your current set up would put you at 2655 @ 60.

and if 235's would fit that would take you 2600. so that would take the difference with od down to 250 rpm at 60. (with the od and the 4.30)

My car with 4.11 and 225 rear tires runs 2930 rpm at 60. I have driven it on road trips to Charlotte NC, Detroit, Vermont, Virginia, etc.

Just did 135 miles on Sunday and got 19 Mpg on that trip. My thoughts are that it would take a lot of miles for any fuel savings to pay for the OD. Might have some effect on wear and tear onthe engine but at 3000 miles of driving per year, again is it significant???

Edited October 5, 2010 by greg g

[Young Ed]

So Fred you might be just fine with a 3.73 and OD. Anyone know what the power curve of these engines looks like? If you get too low in the RPMs I know you can have driveability issues or you might end up never using the OD. For instance with Gregs math at 55MPH Fred would be under 2k RPM

[RobertKB]

My car with 4.11 and 225 rear tires runs 2930 rpm at 60. I have driven it on road trips to Charlotte NC, Detroit, Vermont, Virginia, etc.

Hey Greg, I am surprised you have a 4.11 in your car. My '48 Canadian Dodge D25, same as P15 basically, has a 3.9 as do the two spare differentials I have from parts cars. My '53 Plymouth has a 3.73 as does a spare I have for it. Just wondering why you would have a 4.1? Wouldn't at least a 3.9 lower you RPM quite a bit or at least somewhat? Have your considered switching out the rear gears?

[Young Ed]

My p15 had a 4.11 too. I believe they got delived that way to hilly areas. I have since swapped mine to a 3.73.

[greg g]

My car was sold originally in hilly central PA. You cant get any where there without climbing or descending hills. Plus the fellow I bought it from (original owner) had made some modifications to do his job. He was a farm equipment sales rep. so he travelled a lot of dirt roads, farm lanes, and other off road situations.

When I got the car it had 18 inch wheels and tires on it. the tires were recaps with a diamond pattern Knobby tread. He had a bash plate brased to the oil pan ade from 1/4 inch cold rolled steel plate. a similar deal was bolted to the lower trans mission to bell housing bolts, the protected the bottom of the trans and the e brake.

So he must of done some pretty interesting roads with it. It also had a set of 16 inch rims and a pair of 15's So after I got it I got a pair of 16's for the back and a set pair of 15's for the front. the guy I got the tires from wanted the 18 inch for his truck, so we made a deal. The plate brazed on the oil pan is still there, but I removed the trans gaurd, an its somewhere with the engine compartment lower dust shields.

[55 Fargo]

Greg, my oil pan has 1/4 steel plate welded to it also.

I think for my purpses, and the amouint I drive, about 2-3 k per year, my present set-up with the addition of taller rear tires should fit the bill.

I need new tires, so will try out taller rear tires.

If money was no oject, then yes OD it would be, but at the moment, unless I stumble on an OD trans for cheap, will spend the moeny elsewhere on the car, I could get my bumpers all nice and rechromed, I shudder to think how much my pot metal grille would cost to have re-plated.

The reason I started this thread, is I had never thought about the cons of having an OD trnas, with my present engine and diff.........Fred

[greg g]

Robert the difference between 4.1 to 1 and 3.9 to 1 is only 175 rpm at 60. Probably makes a bit of a difference in 1st and 2nd but I don't know those ratios.

[RobertKB]

Robert the difference between 4.1 to 1 and 3.9 to 1 is only 175 rpm at 60. Probably makes a bit of a difference in 1st and 2nd but I don't know those ratios.

OK, thanks Greg. Not a big difference for sure. Only about 3 rpm/sec.

Interesting about the steel plates added underneath your car and the 18" tires. Your old girl most likely travelled some pretty bad terrain yet lived to tell about it!