Updating an older car with an overdrive transmission…

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Ernie (poster here) asked me about this. In my opinion, the biggest functional disadvantage of owning an older (pre-computer, so  – roughly –  1981 and  older) vehicle is  the  built-in compromise of either  decent  fuel economy/highway legs or  good acceleration off the line. Many of the cars from that era – especially performance cars and 4WDs – came with fairly aggressive final drive ratios  and three or (maybe) four speed transmissions, without overdrive. The result was they  were tire-fryers off the line but  their engines were  screaming at  high RPM  at fairly low highway speeds. For example, my ’76 Trans-Am (3.90 rear gears, no OD transmission) used to tach about 3,300 RPM to maintain 65 MPH; with a redline of 5,400 RPM, that is making the engine work. And drink gas, too.  I swapped in a 2004R overdrive and the RPMS dropped to just over 2,000 at 70 in top gear  – a huge difference. The car is much more enjoyable to drive on the highway and I don’t feel like I’m tearing it up just trying to keep up with traffic. This swap can be done easily and fairly inexpensively with just about any older/classic car and is in my opinionmore bang for the buck that the more common/popular switch to EFI in place of the factory carbs….

Your thoughts?

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6 COMMENTS

    • Hi Scott,

      I’m not knowledgeable about antique Alfas, but someone here probably is. Have you tried any of the Alfa forums?

  1. I WOULD LIKE MORE INFORMATION, SUCH AS PRICE, PERCENTAGE OF RPM REDUCTION AND HOW TO FIT IT TO A 1929 PACKARD MODEL 645 TRANSMISSION.

  2. Depending on your application, a Gear Vendors overdrive unit might be another option. It attaches to the back of the transmission and before the drive shaft, thus requiring a shortened drive shaft, and adds another “gear” to the system. If you have a three-speed, it essentially becomes a six- speed. It can interface with computers for shifting, can be shifted manually, or can work automatically. It let’s you keep your stock tranny and ratios but permits you to have both gears in between the ones you’ve got (if they are spread out pretty far) as well as an overdrive. They can handle high horsepower, too.

    Not a cheap upgrade, but interesting.

    • That is interesting, doubling up your gears! I remember a work truck one of my buddies owned had something similar, but I think it was something located in the rear end.

    • The GV unit is extremely cool, but it’s pricey and that’s the major downside. I haven’t checked ’em out in a few years, so maybe the cost has gone down, but if I remember right, they used to be in the neighborhood of $2,500 or about twice the price of a “built” OD automatic.

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