Manual Transmissions: Electric Overdrive vs. 5- or 6-Speed

Driving my car at highway speeds over 70 mph while the engine turns at 3500 rpm, is not enjoyable.

Even with the windows closed, plus the air conditioning and radio on, it is hard to ignore those eight cylinders which sound like they're ready to blow up at any moment. Even while street driving I find myself trying to shift into a nonexistent 5th gear.

And let's not even begin talking about all the resulting noises and vibrations created by high RPMs. Sometimes it feels like my car will rattle itself apart.

While talking to the folks at Classic Muscle Motor Company in Daytona Beach, I inquired about options, and Randy suggested I look into electric overdrive, so I did a bit of investigating in order to familiarize myself with those systems.

I knew from articles and talking to many car enthusiasts that using either a 5- or 6-speed tranny was a viable option. Needless to say, cost always plays a big role in these decisions but, according to Randy, the cost of the overdrive system is about half of that of a 6-speed transmission.

After some online research, I narrowed down my options to two companies.

  1. Silver Sport Transmissions
  2. Gear Vendors Under/Overdrive

Silver Sport Transmissions, or SST, offers a 5-speed Tremec TKO PerfectFit™ conversion kit for 1968-1982 Corvette C3s which, according to their rep, was an easy swap without the need for any fabrication. The only modification necessary was to drill the firewall in order to install the hydraulic clutch.

I took the opportunity to ask why they recommended a 5-speed versus a 6-speed for my '76 Corvette.

He explained that the RPM drop difference between a 5- and a 6-speed using my 70 mph/3500 rpm example, would be a meager 35 rpm.

In other words, my car's rpm would drop from 3500 to 2240 with a 5-speed, and from 3500 to 2205 with a 6-speed transmission, using the example above. He also mentioned that carbureted engines don't really like excessive overdrive.

The cost for the Tremec TKO 5-speed kit is $4,100 and includes shipping and everything necessary to make it a bolt-on application, including a modified driveshaft. Installation is, of course, extra, and I am guesstimating that the total cost of such conversion (transmission plus installation), would be in the $5,000 range.

My second call was to Gear Vendors in order to get details about their overdrive system.

The first question I asked was if this was an easy/no modification install on a 1976 Corvette, and the answer was no. Apparently, the recommended installation would involve some "heavy dimpling" of the transmission tunnel (76-and newer C3s have a steel floor), in order for the overdrive unit to fit. Plus, the original driveshaft would have to be shortened in order to fit.

When asked about the RPM drop from the aforementioned 3500 rpm at 70 mph, the rep said that their unit would drop RPM to around 2600 when engaging the overdrive by pushing a button conveniently placed on the shifter knob.

The cost for the Gear Vendors overdrive system is $2,895 plus shipping, shortening and balancing the driveshaft (approx. $200), plus installation.

My best guess—depending on how much "hammer time" would be required to reshape the tunnel—is a total cost of about $4,000.

So far, I think the Tremec TKO is a far better deal when you take into account the fact that there's no need to use a BFH to make it fit. Granted, installation is a bit more involved since the console has to come out and the firewall has to be drilled in order to accommodate the hydraulic clutch.

However, money and installation challenges aside, I feel the biggest distinction is the almost 15% rpm difference between the Tremec and the Gear Vendors overdrive unit.

That difference translates into 360 rpm lower for the Tremec TKO, so I am definitely leaning in that direction.

But five grand is quite a bit of cash so, for the time being, this idea shall remain a wish.

And next time I'm on the Interstate, I'll either have to drive at 1976 speed limits of 55 mph to keep my 350 revving at around 2600 or put up with high RPM, noise, and vibrations when pushing the pedal to the current 70 mph.

Thanks for following my '76 Vette Blog!

Product Links... (#sponsored)

• How to Restore Your C3 Corvette: 1968-1982
• 1968-1982 Corvette Restoration Guide, 2nd Edition
• Corvette Black Book | 1953-2019
• 1976 Corvette Service & Overhaul Manual
• 1976 Corvette Assembly Manual

How to Rebuild & Modify High-Performance Manual Transmissions