Re: The Duchess Project: 1940 Super 8 Convertible Sedan

Posted by Joe Santana On 2015/12/4 10:50:13

Thanks, Dave,

But I have news for you. The result is the same, except for being less accurate because I only sample 1 wheel revolution instead 10.

The shaft turned 2 times for 1 revolution of the wheel, after I pulled the jack stand on the passenger side.

If you watch the tail end of that video, he doesn't multiply by anything. He gets 39 shaft turns to 10 wheel rotations and divides by 10 for a 3.9:1 ratio.

But what you're telling me is that my ratio (using my 20.5 rotations) is close to 4.1. The closest ratio is a special 1803 differential with 4.09.

Is the difference that he has Positraction or something?

Looking back at the parts book, p 260, 12.005, 1935-1941 Parts, for the 1803 I see:
900131 3.92 Std 1803 - 6 - 1903 - 6
900065 4.09 Spl " and the 1804,1807
900064 4.36 Spl used for Econodrive with all above

The higher the number, the lower the gear (another issue for me). So my 4.09 differential gives me more speed per engine RPM than the recommended overdrive differential of 4.36.

So I'm thinking I should keep the 4.09 (since I can always kick down into 3rd under if I need more torque up the hill.) And it also means I need Fewer teeth on the speedometer pinion. Like 17.

Edit: Don corrected me. For OD cars with 4.09 the gear is at the bottom of the shaft, so I've corrected the number below:
In fact, on page 49, 3.257, for Speedometer pinion and shaft, I see:
354976 for 4.09 differential with OD, 17 teeth

Don does not have a 354976, so checking around for one, starting with John Ulrich.

Even though I believe my problem with the speedometer is solved, I'd still like to understand your formula, Dave. It is correct, but why? And why is the other guy correct, too?