[55wagoncrazy]

are those specs for a wagon only? My 55 4 dr Belair sedan was a manual and had 3:55s

Those are the way Chevrolet built them. If your rear was 3:55/56 it was swapped at some point. Not all that uncommon to get a used rear end assembly from a salvage yard that just "fit"

 

[Shadetree55]

Those are the way Chevrolet built them. If your rear was 3:55/56 it was swapped at some point. Not all that uncommon to get a used rear end assembly from a salvage yard that just "fit"

Just asking, it is definitely a 3:55 as I bought NOS parts to rebuild it and is factory 3 speed BUT it actually had the factory overdrive cable under the dash that the cable had been cut off. I had forgotten about that. So someone definitely removed the OD and changed the rear end

 

[TheCoastalGhost]

I think this is the right one? BB123?

 

[hotrodg726]

View attachment 461567
I think this is the right one? BB123?

OK on the drivers side is a long part number and a casting date please post that.

 

[acardon]

With a 3:55 rear, 25" tires you will see about 3200 RPM at 65. That won't cause it to heat up. Forget the rear end.

 

[hotrodg726]

With a 3:55 rear, 25" tires you will see about 3200 RPM at 65. That won't cause it to heat up. Forget the rear end.

I disagree with that. if I take my car out of overdrive and put it in 3rd its 3200 at 70 mph after a while the temp comes up 15 degrees.

 

[hotrodg726]

View attachment 461567
I think this is the right one? BB123?

I believe this codes to a 3.55 ratio built the dec 3rd

 

[hotrodg726]

the top chart is 3.55 ratio the lower chart is 3.08 ratio big change with a rear gear change.

 

[TheCoastalGhost]

appears to be A1862, 3725899

 

[hotrodg726]

that's a January 18, 1962 differential and codes out to a 3.36 ratio. below is the rpm vs mph chart for 3.36

here is the link I used to determine ratio based on the casting date. 1958-1970 Full size rear Differential Identification
this makes more sense if you were keeping up with traffic probably going faster than you thought. If you speed isn't right or not working I would suggest a gps app on your phone.

 

[acardon]

the top chart is 3.55 ratio the lower chart is 3.08 ratio big change with a rear gear change.

Doesn't mention tire size. Makes a big difference. Link to calculator, even if it doesn't say that.

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[TheCoastalGhost]

that's a January 18, 1962 differential and codes out to a 3.36 ratio. below is the rpm vs mph chart for 3.36
View attachment 461582
here is the link I used to determine ratio based on the casting date. 1958-1970 Full size rear Differential Identification

That seems to check out with my rpm range keeping up with traffic. So…is a gear swap needed?

 

[hotrodg726]

Doesn't mention tire size. Makes a big difference. Link to calculator, even if it doesn't say that.

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RPM/MPH From Gear Ratio Calculator by Wallace Racing
I used the OP's stated tire height of 25"

 

[acardon]

That seems to check out with my rpm range keeping up with traffic. So…is a gear swap needed?

Not to fix your getting warm problem.
I drove a completely stock 1955 Chevy with 3:55 gears from 1957 to 1968 for over 200,000 miles in Texas with most of it between El Paso and DFW and it never overheated once. I remember watching the temp gauge going over the Guadalupe Mountains (8,000'). It would go past the center mark but never boiled over.

 

[Beach-n-cars]

That seems to check out with my rpm range keeping up with traffic. So…is a gear swap needed?

Download "speedbox" for your iphone. its a GPS speedometer for your phone.
drive on highway at say 60 mph, watching your phone.
record RPM at 60 mph
Plug numbers into this to determine rear ratio.

Calculate RPM for Given Speed(MPH), Rear Gear Ratio, and Trans Gear Ratio

Calculate RPM for Given Speed(MPH), Rear Gear Ratio, and Trans Gear Ratio

purperformance.com

or jack the car up, do the mark the driveshaft and tire thing.

Do you need a gear change? thats entirely up to you. but if you know your gear ratio, you will be able to figure out what kind of RPM you will hit at what every speed you like. then determine if listening to your engine at that RPM for a long trip (like i think i read you have planned) will be to your liking.

 

[Dragsix]

Depends. What you said in the first post was 3100-3500 rpm at 60-65 mph. 3500 at 65 is in my view too high. 3000 at 65 would be a different story. So what you might need to do is double check the rpms vs speed. As for speed, you will not be able to rely on the speedo. It is not accurate with the change in gears and tire size so you will probably want to use a gps app or something to nail it down. If the rpms fall within a normal range vs mph, then the problem lies elsewhere.

 

[TheCoastalGhost]

Yeah like I said it was a rough guess since my speedo doesn't work. I'll see if I can get a digital speedo or an app and figure it out from there (after I put a new thermostat in)

 

[TheCoastalGhost]

By the way, it was so hot that day my phone overheated, so i'll have to figure out a new dash mount configuration loll

 

[hotrodg726]

That seems to check out with my rpm range keeping up with traffic. So…is a gear swap needed?

I would but it's not mandatory. in your current situation flushing the cooling system, using a 60% water 40% antifreeze mix with a coolant additive, and proper ignition timing should have you in a good spot.

 

[Shadetree55]

Yeah like I said it was a rough guess since my speedo doesn't work. I'll see if I can get a digital speedo or an app and figure it out from there (after I put a new thermostat in)

as stated---get the speedbox app for your phone or---

Amazon.com : gps speedometer