rear gear swap

4

48buickkid

Member
I have a 48 Buick Roadmaster with the Dynaflow transmission if i remember right the current rear gear ratio is a 4.10. i would like to swap to something to get better gas mileage once i get it on the road. what is the lowest road gear that will swap in? and will anything gear wise from the smaller series work are their rearends the same as the bigger series? im not looking for acceleration so far the lowest i found was a 3.60 from a manual trans 48 Roadmaster .i know i could do a 700r4 swap and use a Chevy truck rearend to keep the 5x5 bolt pattern but that is out of my budget right now and trying to keep the reasonably original drivetrain wise. any help would be appreciated
 
If I remember correctly the gears were the same up to '55. This means you have available to 3.36, maybe lower. I don't think your stock gear is 4.10. I think it's close to 3.90 or 3.60. Take the rear cover off & look at the ring gear. It will have the ring & pinion gear tooth count on it like 13-47 or.

Tom T.
 
Keep looking, any rear gears between 1940 to 55 should fit. Have a friend that has delt with many Buick differentials bakntheday, and he says that he had found many different gear ratios in all kinds of different series Buicks. Its possible but not usual for a 4:10 ratio in a dynaflow, anyway they are out there, I just got a 3:61 from a 53 special for my 39 Century. Any of the taller ratios will give you better drivability and mileage. Most are being offered from guys who are wanting to get rid of all the leftover parts from a driveline swap or just parting out and they usually want to get those big parts out of the way. a complete rear end with driveshaft would make it a lot easier for you. [if its the same length] There is a 3:43 ratio available, probably in a 55 model.
 
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ill check my 48/49 shop manual again see which ratio it says again. But thanks for the info is there any kind of compiled list of rear gear ratios for 40-55 cars by model and trans?
 
49 rear end ratio

My 49 Roadmaster had a 4:11 and i got a 52 roadmaster rearend w/ a 3:61 and other than 2 small holes to drill in the x member plate for the P-Brake it was a easy swap. and it is now a nice cruiser at 65 to 70 mph. I do have a 200R4 and looking for a mid 80 . olds rear end which is 5 on5 and just 1/2 in. less in whit to get rid of leaks and whine and a faster take of and lower rpm at cruise. I also did a duel carb.manifold/single exhaust and a 52 head w/higher comp. and hardened valve seats.
 
my car is late 48 dynaflow, the hydraulic lifters are the signifier that it is the late year run which did have 4.10 gears for anyone wondering i will give the specs from my manual on all the 48-49 cars

series 40-50(syncro mesh 1948 and 1949)
standard gear ratio 4.45, 49-11 tooth count
optional gear ratio 4.10, 41-10 tooth count
series 50(dyna flow 1949) 4.45, 49-11 tooth count

Series 70(syncro mesh 1948 and 1949)
standard gear ratio 4.10, 41-10 tooth count
optional gear ratio 3.60, 51-14 tooth count
series 70(dyna flow 1948-1949)
First part of 1948 3.90, 51-13 tooth count
last part of 1948 and all of 1949 4.10, 41-10 tooth count

hydraulic lifters are the indicator of late 1948 dynaflow cars an oil line with go from the filter housing to the front of the head and feed the valvetrain. all 1949 dynaflow cars have this
 
well be careful of going to high on the gear as them old buiks, won,t handle too much, but 4.10s are too low.
 
39Cent,
Is there a list of the different driveshaft and torque tube lengths for the 41-55 Buicks?

Thanks!
 
JR9162 said:
39Cent,
Is there a list of the different driveshaft and torque tube lengths for the 41-55 Buicks?

Thanks!
Click to expand...

I havn,t seen any driveshaft length list, maybe someone out there knows. but as far as I know the 1940 to 55 differentials are the same size and can be interchanged, driveshaft lengths may differ. Just for information those diffs can be adapted to 1939's and below by machining off about .250 from the bolting face, and adapting the 39 driveshaft and torq tube.
 
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i wouldnt run into any carrier problems due to the increased height of a say 3.10 would i? i know some of the later style gm carriers differ and can only hold a certain set of gears does anyone know if this happened with the buick rear ends?
 
48buickkid said:
i know some of the later style gm carriers differ and can only hold a certain set of gears does anyone know if this happened with the buick rear ends?
Click to expand...

Yes, it did, but this is a problem only if you try to swap just the ring and pinion. If you do this, you will also need to swap the differential case (the part the ring gear is riveted to). Besides, you really don't want to try to remove and replace those rivets anyway. If you swap the complete center section, everything fits as-is and you won't need to set up the pinon depth, which can be a real pain.

The tallest stock gear that will fit this rear end is 3.42, which was used on 54-55 Centurys, Supers, and Roadmasters with Dynaflow.

Ray
 
Rear swap

The torque tubes from 42 to 48 are the same for Roadmaster and the Super, I can't say anything about the Special. The 49 torque tube was a different lenght. I took 3.42 out of my 48 Roadmaster to replace all the bearings, now Buick is a stick. All I did was remove the entire rear from under the car. I unbolted the rear from the torque tube assembly, pulled the axles, pulled the gears from the housing, and replaced it with a set of 4.10's that I took from my Dad's 42 Roadmaster. I'm just waiting to get the ambition to do it all over again. I was getting 17 to 18 MPG with the 3.42 on the highway. These 3.42's are from a 55 Roadmaster

Anthony
 
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