Can I tell what rear end I have with this 73 RR fender tag?

[b_body_hopeful]

Here is my fender tag. I read on other threads that one could tell what rear end was placed in a car just by decoding the fender tag. I did that already, but I could not find what rear end of ratio gears I have. No tag on diff or anywhere else on car. I have a 73 RR with 400 cu. in and 4 speed hurst shift. Thanks in advance.

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[Badvert65]

First place I would look is on the rear end. Factory put a metal tag under one of the cover bolts (or under one of the bolts holding the center section in - on 8 3/4, which you likely have). Tags don't always survive but it won't hurt to look. If you have a 3.23 ratio, it will say 3.23 on the tag. Pretty cut and dried there.

 

[PlumcrazyRR]

Yes, you can tell what came in there from the factory by decoding the fender tag in a round about way. Since it was a 4 speed, that means it started out coming with the standard 3.23 and since it is not optioned out for an A36 that let's you know it did not come with a 3.55 sure grip. What is actually in the car right now is another story. It could have easily been changed over the years.

 

[HawkRod]

Hi,

There are a number of decoders on the web. I ran your tag through one and this it what it came up with:

RM21:
Plymouth Belvedere,Satellite
Medium, Road Runner
2 Door Coupe

P3G: 400 260HP(net) 1-4BBL 8 CYL
1973
St. Louis, MO, USA

221968: Sequence number

E68: Unknown
D21: 4 Speed Manual Transmission
FE5: Bright Red Exterior Color
C2L3: Trim - Charger, Vinyl Bench Seat, Parchment
HL2: Unknown Int. Door Frames
517: Build Date: May 17
347144: Order number

FE5: Bright Red Top Color
U: USA Specifications
A88: Interior Decor Group
B41: Front Disc Brakes w/Standard 10in RR Drum
G36: OS Dual Racing Mirrors
H51: Air Conditioning with Heater

J52: Inside Hood Release
M21: Roof drip rail moldings
M26: Wheel Lip Moldings
N41: Dual Exhaust
N42: Chrome Exhaust Tips
R11: Radio Solid State AM (2 Watts)

V8W: Tranverse Stripes, White
V9W: Unknown
28: Unknown
END: End of Sales Codes


As stated earlier, look for a small brass tag on one of the bolts. Mine also clearly had 3.55 in yellow crayon on the rear of the differential, so if you carefully clean yours off, you might get lucky.

Failing all that, jack the rear wheels off the ground. Spin on wheel one full circle and count how many times the differential rotates. You should be able to see the difference between about 3 1/4 (3.23) and 3 1/2 (3.55) or almost 4 (3.91). I suggest you align a u joint to a known point and count from there. Also, if the other tire spins the opposite way, you have a sure grip.

Hope this helps.

Hawk

 

[Photon440]

Would this be an 8.75 or 9.25?

 

[PlumcrazyRR]

Would this be an 8.75 or 9.25?

8.75

 

[mcmlvii]

Also, if the other tire spins the opposite way, you have a sure grip.

Isn't it the other way around, with a sure grip, they'd both spin the same direction and an open diff they'd spin opposite?

 

[b_body_hopeful]

X2. I thought that also. Thanks to all. Will do what everyone suggested. Trying to do this without removing everything. Thanks again

 

[WP29440SE]

Also, if the other tire spins the opposite way, you have a sure grip.

Hope this helps.

Hawk

if the wheels spin the opposite direction , that would be an OPEN rear end, one wheel peel

 

[HawkRod]

Also, if the other tire spins the opposite way, you have a sure grip.

Hope this helps.

Hawk


if the wheels spin the opposite direction , that would be an OPEN rear end, one wheel peel

Doh! Sorry guys - wrote that backwards by mistake. Must have been distracted, drunk, dimwitted, whatever...

Thanks for the correction and sorry for the mistake :iamwithstupid:

 

[monaco66coupe]

I found if open, double what you see. One full wheel rotation gave me a little more than 1.5 at the U joint. which told me 3.23. 1 /34 turns approximated 3.55
Just a tip.

 

[BeepIn']

A 73 Road Runner should have an 8 1/4 rearend

 

[PlumcrazyRR]

A 73 Road Runner should have an 8 1/4 rearend

Yes, if it came with a 318 or a 400 2 bbl, but not the 400 4 bbl. The 400 4 bbl came with the 8 3/4.

 

[69Coronetrt]

A 73 Road Runner should have an 8 1/4 rearend

If you look on the data book page posted above, (Axle, rear-ring gear size) I'm sure it's pretty clear the Satellite model with anything more than a 400-2bbl got the 8.75.

 

[Badvert65]

I once had a parts car 1973 340 4 speed Road Runner that came with a factory 8 3/4. Could be that the stick cars got it along with big blocks.

 

[Chargerjase]

I can only talk from a 73 Charger pov, but mine has a 8.25 and has a big block 400 in it. I think the 8.25 was standard for 2bbl while the 4bbl came with an 8.75 minimum.

 

[monaco66coupe]

I had a 69' charger 318 V8 with the 8 1/4 as well. I didn't care for it. My 78 magnum rear also clunked all the time.

 

[HawkRod]

A 73 Road Runner should have an 8 1/4 rearend

Yes, if it came with a 318 or a 400 2 bbl, but not the 400 4 bbl. The 400 4 bbl came with the 8 3/4.

Let's not forget about the 340s. My 73 Road Runner has a 340, 727 and a 3.55 suregrip 8 3/4 rear.

 

[PlumcrazyRR]

Let's not forget about the 340s. My 73 Road Runner has a 340, 727 and a 3.55 suregrip 8 3/4 rear.

I was using his 440 4 bbl as a reference, because that was the car we were talking about. If you notice, I did not include 340's in the 8 1/4 category. Bazinga! But thank you for clearing it up for everyone else. Not trying to say this in a jerk way, by the way, because I was not entirely clear in my original post.

 

[b_body_hopeful]

Thanks for the replies, this is why the question......I am running 235 X 15 X 60 for the rear and when going down the road at 70 mph, my tach reads 3000 rpm, and the engine sounds like SSSSuuuuucucccccckkkkkkkk gas. (LOL you know what I mean) Is this about the correct rmp for 70 mph? WHat did you do, if anything, to lower your rpm and save some gas?