VCCA Home Forums TECHNICAL - 1945 AND NEWER 1946-1948 48 Fleetline Aero Ring and Pinion Swap

Hi,

Anyone ever swap in a new 3.55 :1 RATIO RING AND PINION into the existing rear axle housing into a 48 Fleetline Aerosedan?

I was wondering if this is a better option so I can keep the car looking more original but still be able to cruise on the freeway a bit easier.

Or is it better to swap in a later rear axle with the gears in it already? If so what rear axle do you guys recommend?

Thanks

Ptechniker48

Might enjoy reading this. info

FYI, body tag decode,

3 = St Louis
F = '48
K = Fleetline
L = December so very near the end of '48 production.

The option in 1948 was a 3.73 ratio. Reduces engine speed by 10 %, A 3.70 ratio can be found in al 1953 and 1954 cars with a standard trans. Makes a bigger improvement that you would guess.
The 1950-1954 Power Gildes had the 3.55. Reduces engine speed by 15% and takes away some of the hill climbing power.
Either one makes cruising at 65 MPH a breeze + increases fuel mileage.

Great info on my body tag code ruscar!

Looks like I got one of the last 48's.

Chev Nut, does this mean I got the 3.73 ratio?

Hi Art,

I suggest that you will probably have to do the old "count the number of shaft revolutions" trick to determine what ratio you have.

If you have a 3.73 I will strongly suggest that it is not worth the cost and effort to change to a 3.55 however you do it. That is only about a 5% difference. My '37 Master coupe does very well at 65 with the 3.73 in it.

And I am not sure you really want to go much faster in a 70 year old car unless you have modern suspension and brakes.

The differential number will determine what ratio you have. It is STAMPED into The rear axle housing , right side just behind where the torque tube goes into tne housing and above the side center rib on the housing Will be two letters with four numbers after it.
Iti s a very rare option and most commonly found in cars sold in the great plains area.

Is there any other way to determine if I have the 3.73 or 4.11 ratio gear? I notice the specification manual/heritage kit PDF categorizes the 3.73 with a heading of RPO* and the 4.11 with Reg. Curious what gear ratio most 48 Fleetline Aero owners have.

I was cruising around 55 this weekend on the freeway and all seemed good. Either way, I bet its a good idea to change out the gear oil and get a new U joint installed just for good luck! Maybe a clutch adjustment too!

Thanks
Art

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What's the old saying, If it ain't broke, don't fix it! If it's not making a noise, nice and smooth, leave it alone. New replacement parts are not what the use to be.

You would be very lucky indeed if you have 3:73. Mine had the 4:11. I did as described in the info link I provided in my post above. I got the "pumpkin" fron a "53 stick (3:70) and swapped it out.

One side note, you might want to get a separate insurance policy on your sun visor if you start cursing around 70 mph..

Glad to see your back sharing your knowledge Russ.

dick

Going back to 1948 the owner never ordered the car he wanted. He would pick a car out of the dealers stock. Also in 1948 Chevrolets were in high demand and owners were just lucky to find one. and the Fleetline Aeros were hard to come by due to being so popular.
There is no other was to tell if the car has a 3.73 other than removing the cover and counting teeth. There would have been a order sheet glued to the windshield but that got thrown away on clean--up.
Owners usally drove between 55 and 65 MPH back then and never though or even knew a different ratio existed.
The 4.11 gave good acceleration in high gear between 10 and 60 MPH, where most driving was done. Over 65 it was begining to run out of breath. Also often people would start in 2nd gear where with a 3.73 a little clutch slippage is ecessary.
I would be 99 and 9/10 % sure you have a 4.11.

Rus........great info! Duly noted, I will definitely not be doing more than 55 on the highway.

I appreciate the advice and now plan on not screwing around with the gears.

Chev Nut and Rus thank you for the help! I just love this forum, you guys rock!

Art, if you do get the desire to go to 3:55 gears, use a 3:55 CENTER SECTION AND TORQUE TUBE out of a '50 to '54 POWERGLIDE .......... instead of going to a late model 3rd member.

Going to a late model 3rd member is a domino effect, meaning a late model rear end constitutes a late model transmission, which then requires an engine to transmission adapter, which then leads to a 12 volt conversion, etc.

What years of Chevrolet have the differential number stamped on the rear axle housing, and what do the two letters and four numbers refer to?

Don't know what years but probably most. The first letter stands for the year. The second letter is code for the plant that made it. The first and maybe the second number stands for the month and the second and third is day of month. At least that is the way it is for my 1948.

If the LAST letter is a L or M it is a 3.70 ratio from a 1953-1954
If the last letter is a S or T it is a 3.55 ratio from a 1950-1954 Power Glide.
For a 1948 with a 216 engine the 3.70 is a excellant choice and was an optional ratio in 1948.
The 3.70 will make 65 MPH smooth and quiet and will be some power loss on hills. The 3.55 will reduce engine speed by 5% more and lost of power on hills more noticeable.