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Hi All:

I need to find out what the standard differential ratio is for our '41 120 coupe.

I think an OD car was 4.09, but the non-OD car is what I need. We are heading across country to run the Hemming's Challenge in Bowling Green, KY. All my reference info is home in Oregon. The car is in Chattanooga.

We are getting new shoes for her, and I need to recalc our rally speedo settings. This is a time-speed-endurance rally and is calculated to 1/100 second. I need to "rough" calc the speedo settings so we can then adjust for heat, altitude, and tire pressure.

Thanks for your help!

Pat

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The ratio should be stamped on the edge of the center pumpkin. The numbers are small and are probably covered in dirt. Do some test runs with a GPS and set your speedo from there.

4:36:1 is the number you are looking for

Thanks for the comments. Unfortunately, setting by GPS is not accurate enough (we calibrate out speedo to .01 mph). We have to build performance tables to calculate time lost on braking, acceleration, speed change, and turns.

It would be easy to look at the differential if it was here, but it is several hundred miles away.

I do not think 4.36 is correct. OD cars had a lower ratio (higher numerically) than non-Overdrive cars. Two web sites I could find said the Overdrive cars had a 4.09. Therefore, the number should be less than 4.09.

Could it be the web sites are wrong and non-OD cars are 4.09 and Overdrive cars are 4.36?

Thanks again,

Pat

Pat,

My '41 120 Coupe with OD is 4.36:1. 4.36 is marked on the right front flange of the pumpkin. When we pulled it out the number of teeth on the crown wheel and on the pinion were marked on the crown wheel, 48 and 11 respectively = 4.36:1. Presently my mate is working on rebuilding a diff for a '39 120 with OD that has 4.54 stamped on the pumpkin flange but when opened up had a 4.36:1 set in it! So who knows.

I suppose the other method is to jack up the rear end and rotate the driveshaft counting the number of revolutions needed to turn a road wheel once.

Hi Ozstatman:

Yes, I could jack it up if it was not a few hundred miles away! I actually have a spare at home, again about 2,000 miles away. There were options and, as they say "I'll know it when I see it"!

I seem to remember that the pumpkins were interchangeable for several years.

Info on the net says that an overdrive car should be 4.09, but who knows! We are running an overdrive with a non-overdrive rear end. Really wants to run!

Thanks for your help.

Pat

According to my shop manual : for a 39 120 w/o od the ratio was 4:36 w od 4:54 std and 4:7 opt. Usually the std ratio for an OD car was a lower (higher #) rear end. I realize that my info is for a 39 but I don't think that Packard changed their thinking in 2 years. FYI my 38 coupe (120) has a 4:36

For 41 mine shows
4.09 std
4.36 std with OD
4.54 optional

I think the 4.09 without overdrive and 4.36 with overdrive must be correct. Overdrive cars almost always have lower geared rear ends for better acceleration, then use the OD to compensate and give better top end. A 4.36 with OD would give an equivalent of about a 3.40 without overdrive. A nice cruising ratio.

Pat

OOPS!

Math error. The Borg-Warner R-10 OD is .70 ratio. The 4.36 would therefore perform like a 3.05. That's what happens when you try and do math in your head! My 4.09 performs as a 2.86.....that's why she wants to fly in OD.

Pat