Changing ratio on a 47 Rear End

[Guest Wes]

I saw this posted on this WEB site about a 47 Packard for sale and was wondering if anyone else has tried this and where can you get the parts to do this?

"Legendary NorCal vintage car mechanic Wayne Ballerstein, who worked in the Oakland, CA Packard dealership in the late '40s rebuilt the 3.92:1 differential which we substituted for the factory 4.09. This was often done at Packard, Hudson, etc. dealerships in the day for drivers who enjoyed frequent fast long-distance business trips over the increasingly better roads after the war."

I'm still working on rebuilding mine and this looks like the better way to go.

Wes

[Guest imported_Speedster]

Are Gear sets difficult to find for the '47 ?

[Owen_Dyneto]

Have you sat down and calculated how much this change will lower your engine RPM at a given speed, say 60 mph? The arithmetic is pretty basic grade-school stuff and I went thru it looking at possibilities for my 34 Eight. I'd guess without doing the numbers that going from a 4.09 to a 3.92 is probably only going to make a difference of about 150-200 rpm. I suppose every little bit helps and finding an installing a factory-correct overdrive is a lot of work, but the reward will be significantly greater.

[Clipper47]

If I am not mistaken the 4.09 was used when the car came equuipped with O/D and 3.92 without O/D. It might make a small difference but not worth the effort IMO.

[Guest Wes]

Thanks for all the good information. Just asking, never hurts to ask.

Wes

[Steamcar]

To help make the decision, here is a formula that will give RPM for a vehicle at a chosen speed.

RPM=(MPH X gear ratio X 336) divided by tire diameter

[Phils38cpe]

I'm in the process of installing an R11 OD tranny in my 38 coupe. According to all the information I can find when the OD is engaged it will give me the equivalent of a 3:13 from my stock 4:36 rear end.