[Vwdiesel] Re: overdrive

[Sandy Cameron]

At 09:32 AM 25/05/04 -0700, you wrote:

>Overdrive to me means that propshaft turns more RPM than engine crank.
>
>High gear say  # 3  means straight through.  1 to 1. RPM.
>

Yes, I had a 1937 DeSoto (Chrysler) with a 3 speed transmission, and a
planetary gear overdrive stuck on the back end of it. The 3rd gear was
straight through, 1to1, to the drive shaft, but the overdrive made it about
.89, like the difference between 4 and 5 on a VW, not a lot, but made a
tremendous saving in fuel, cruising in flat country.

I liked it so much, that when I moved up to a 48 dodge coupe, I scrounged
junkyards until I found an O/D transmission (always from a big, heavy
highway cruiser, chrysler, desoto, dodge sedan) for it, and then moved it to
a 54 dodge sedan I got later.
Nice thing about those cars (and volkswagens) was that everything they made
that used the flathead 6 bolted up without pain, from about 1934 onward.
they even made the 3 speed trannys with an extension equal to the length of
the O/D unit, so the original drive shaft would work. That all ended with
the slant 6 and hemi.
 
The old long stroke straight 6 flat-head, side valve engine had a broad
torque curve, peaked around 1500 rpm, and high engine rpm's did not get much
more power out of it. Just stretched rods and cracked top rings (been there,
done that).

Probably 2 reasons for higher ratio rear ends, one, for a town car with
frequent starts and stops, it's a better match, and, performance is better
if limited to a 3 speed box.

Also for jeeps and trucks, high ratio rear ends give more low speed power at
the drive wheels for hard pulling, but sacrifices economy when the going is
easy..

The trade-off is higher rev cruising, with greater engine wear and fuel
consumption.
Hence, economy cars with smaller engines and 5 speed boxes..

Sandy