'41 Fat Bottom Girl Posted April 26, 2019 Report Share Posted April 26, 2019 I will be rebuilding my 218 Flattie 6 soon on my D19 coupe and would like to be able to cruise at 70 mph to be kind to the mill. I would like to keep my 3 speed original tranny. I went through it and the clutch about 15 years ago and it is in great shape and has had low miles since. Was hoping someone might have done a similar swap out on a rear end and could give me a recommendation of what rear end to look for. I don't mind losing a bit on the low end, as first is a really low gear anyway. I hope to have a minimum of retrofit, maybe fabbing and welding new mounting plates to mate up to the leaf springs, and the drive shaft mod for the U joint. Would like to keep my wheel bolt pattern too if I can. Do you think I can find a rear end out there that will do that for me? Thanx, fellas! Quote Link to comment Share on other sites More sharing options...
Plymouthy Adams Posted April 26, 2019 Report Share Posted April 26, 2019 (edited) your car is geared for the torque and rpm range that was prevalent for the era....you can change the rear ratio but you will need to keep in mind the terrain you will traverse, your basic driving style of the vehicle. If you get to high a ratio you will constantly be in and out of high gear in low rolling hills, bridges and some overpasses depending on load and speed. Even with factory OD they changed the rear ratio lower than that of non OD. Many here seem to think 3.55 is a good gear for use with the flathead. my 54 is a slightly larger mill, little higher CR etc....stock it is 3.73 rear gears and runs 80 with no problem, it will run faster but I have not done the shock upgrade so it floats like a boat at that speed....BUT I do have the first automatic, torque multiplication. My 41 bz cp is running a 2.72 rear gear....but apples and oranges as it has a 383.....lol Edited April 26, 2019 by Plymouthy Adams Quote Link to comment Share on other sites More sharing options...
greg g Posted April 26, 2019 Report Share Posted April 26, 2019 what size rear tires are you running? I have 225 75r 15 on my 46 with 4.11 rear. 62 mph is 3200 rpm. pretty close to where they pulled peak hp. I have run many highway miles at that speed with no ill effect. but last year I did install and od. 65 mph is now around 2500, the combination of the of and 4.11 with those tires calculates out to about 2.90 to 1 final drive ratio in third. Quote Link to comment Share on other sites More sharing options...
Robert Horne Posted April 26, 2019 Report Share Posted April 26, 2019 12 hours ago, '41 Fat Bottom Girl said: I will be rebuilding my 218 Flattie 6 soon on my D19 coupe and would like to be able to cruise at 70 mph to be kind to the mill. I would like to keep my 3 speed original tranny. I went through it and the clutch about 15 years ago and it is in great shape and has had low miles since. Was hoping someone might have done a similar swap out on a rear end and could give me a recommendation of what rear end to look for. I don't mind losing a bit on the low end, as first is a really low gear anyway. I hope to have a minimum of retrofit, maybe fabbing and welding new mounting plates to mate up to the leaf springs, and the drive shaft mod for the U joint. Would like to keep my wheel bolt pattern too if I can. Do you think I can find a rear end out there that will do that for me? Thanx, fellas! I went through three 3 speed trans before I gave up on stock transmissions. First went on all three of them. With a stock 3.9 rear, highway speed was a no no. The 3.9 rear with my 5 speed install, still was too low of a setup. My 3.7 rear with my 5 speed install, is still a little low even in hilly country of Virginia, but does good on the top end. Next, 3.5 rear with my 5 speed, see how that feels........... Quote Link to comment Share on other sites More sharing options...
pflaming Posted April 26, 2019 Report Share Posted April 26, 2019 Robert, which transmission with the 3:73 in my B3B. I want a quieter engine at 65 and better hill towing. Quote Link to comment Share on other sites More sharing options...
'41 Fat Bottom Girl Posted April 26, 2019 Author Report Share Posted April 26, 2019 Thanx for the replies. Greg G., I am running P225/70R15 tires. My third gear ratio is 4.3 : 1 and my rear end ratio is 4.1 : 1 per my shop manual. These P225/70 rear tires turn 736 revolutions per mile, while your P225/75 tires turn 713 revs per mile- If my figuring is right, going to 75's evidently not enough of a gain for me, would only drop my engine revs about 3% with a tire change... Even going to R16 tires wouldn't be a big difference, those at 710 revs per mile. To get overdrive, did you go the T5 route? Quote Link to comment Share on other sites More sharing options...
Plymouthy Adams Posted April 26, 2019 Report Share Posted April 26, 2019 think you are reading your book a bit wrong......third gear should be 1.00 to 1 and while your rear gear as a business coupe should default to 4.1 so overall in the high gear your final drive would be 4.1.....if third gear were 4.3 and the rear was 4.1 your effective ratio would be 17.63 to 1...while I would be surprise you could get to 18 MPH you could however pull the empire state building behind you.... Quote Link to comment Share on other sites More sharing options...
'41 Fat Bottom Girl Posted April 26, 2019 Author Report Share Posted April 26, 2019 Thanx Plymouthy! You are absolutely correct about third gear... 1:1 it is! a Duhhh on me! Quote Link to comment Share on other sites More sharing options...
greg g Posted April 27, 2019 Report Share Posted April 27, 2019 (edited) I picked up two borg warner r 10 cores and had George Ashe make one good one. then added it to a 56 three speed. same gears but better synchros. he gave me a credit for the unused case and solenoid and input shaft not used. Edited April 27, 2019 by greg g Quote Link to comment Share on other sites More sharing options...
'41 Fat Bottom Girl Posted April 27, 2019 Author Report Share Posted April 27, 2019 Greg G- Thanx. Gives me another idea to research. You folks sure are a wealth of info! Am checking out adapters too- flat 6 to different trannys. This will be fun checking out the different options. The solution is out there! (wow, did I just discover my quote?) Good info... Quote Link to comment Share on other sites More sharing options...
greg g Posted April 27, 2019 Report Share Posted April 27, 2019 you can also consider newer rear axles. Cherokee, Ranger, five bolt early Dakota, Explorer, mopar B body. better brakes ratios from the 3.30's to 3.75's. takes some spring pad work and shock mounts, and drive shaft ujoint work but doable at home if you can weld good. Quote Link to comment Share on other sites More sharing options...
'41 Fat Bottom Girl Posted April 27, 2019 Author Report Share Posted April 27, 2019 Thanx Greg G- I will look into this for sure. That helps 'cause I had no idea what rear end to look for while doing more research into the different possibilities. Good info! Quote Link to comment Share on other sites More sharing options...
Adam H P15 D30 Posted April 28, 2019 Report Share Posted April 28, 2019 Don’t forget the venerable 8” Ford. Early 65-66 mustang or 64 and up Falcon. Mavericks too but those get a little pricy. Sometimes you’re better off looking for modern drum brake rear axles, add discs to the front and it keeps the pedal effort down without a booster verses all discs. Quote Link to comment Share on other sites More sharing options...
'41 Fat Bottom Girl Posted April 28, 2019 Author Report Share Posted April 28, 2019 Great additional info. Thanx much! This is a good education here... Quote Link to comment Share on other sites More sharing options...
40phil41 Posted April 28, 2019 Report Share Posted April 28, 2019 My '41 Dodge has a 3.73 pumpkin that I installed from a 55 Dodge. Not a direct swap since the axle splines are different between the two years. My '40 now has an overdrive transmission. Both options work great at 65 mph. Phil Quote Link to comment Share on other sites More sharing options...
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