Add overdrive to C6

[charlie D]

I'm considering putting a Gear Vendors overdrive on my 67 GT500 C6 automatic. The stock rear is a 3:50 and while that is great for stop light races, it keeps my engine screaming at today's highway speeds. I am still using 15" tires with the 70 profile so that is still a stock setup. Travelling 400 miles on open interstate with even the 18 wheelers rolling at 80 puts me truning well into the mid 3000's I'd like to add the extra gear just to make those segments of travel more comfortable. Anyone done this? If so, are you happy with it? Any information welcome. Oh, yes, I will save and store any removed parts.
Charlie D

[TA Coupe]

I have one in my 93 Supcharged Lightning Supercab. Give me a call and I can give you some insight. Don't feel like typing for 10 minutes on my phone.

    Roy 510-917-FORD (3673)

[roddster]

  10/16/23
    I have recent experience on installing a GV overdrive onto the C6 trans in my Tribute car.  It is an S code GTA.  Well, no longer is the 390 in there.  It now has a 428 CJ block, aluminum heads, and CJ exhaust manifolds. It really does not matter what the engine is built for.  Hot Rod magazine shows a GV on drag cars often enough.  And, the transmission is very similar to the C4 trans so the GT 350 owners can read this too. All the car changes start at the rear of the transmission.
  1) Order online.  The next day you'll get an email from GV about what gear is in the rear end.  There is a special speedo
      sensor they supply that tells the GV control unit what speed the car is going.
  2) Thats when they tell you the delivery schedule.  My order made in May was 13 weeks.  I had plenty to do on the car, so
      that was not an issue.  It may be an issue for others.
  3) The overdrive unit bolts on right where the C6 (and C4) extension housing bolts to the main transmission body.  No
      modification required.
  4) The GV unit is longer.  Way longer.  You will need to either shorten your cars driveshaft, or order a new driveshaft to the
        correct length.
  5) Wiring is simple.  Follow the instructions in the installation manual.  I did not get the cute little control box GV shows.  I
      did get the switch and the indicator lights.  This was OK with me.
   6) The issue: on stock exhaust systems, the twin pipes at the back end of the crossover pipe point right at the overdrive
        unit, and too close to the unit too.  Custom exhaust work will be necessary. in the area behind the crossover.
   7) Cost (no need to ask) $2995.00, plus shipping (72 LBS) and sales tax.  Might even be more now.  Plus the driveshaft
       work, plus the custom exhaust work.
   
     My comments: you'd think that they would have gone through the engineering to just have the overdrive section bolt on   
     to the trans right where the extension housing was removed.  As it is there is an adapter that is at least 18" long moving the OD unit farther back.  I get it.  Too many different applications.  That section simplifies the installation.
    Then there is the maintenance.  A required, every 8000 to 10,000 mile fluid change as the GV unit has its own fluid pump.  The unit uses Dextron or a Dex/Merc fluid combo.  Synthetic fluid in NOT recommended.
   GV says that I should end up with an approximately 2.80 ratio.  I have a 3:70 limited slip rear in it. I have not yet driven the car.

[Royce Peterson]

Cropped and Resized. Still a very tiny photo.

[Brandon]

I've installed a GearVendors overdrive on 3 vehicles.  They work great and install is fairly simple, but besides the price, I've run into 2 issues:

• As roddster mentioned, the GV unit is much longer, so much longer that I've had to use pinion shims to raise the angle of the rear-end more than once.
• While it shifts well, at some point in time, they had a real electronics problem with their control box.  It assumed 12V, and once you gave it 12V, if you then dropped to somewhere between ~5V and ~11V IIRC, it would go into a bad state and stop responding/shifting.  So you'd crank the car to start, sometimes the voltage to the control box would drop below 11V, and then it wouldn't engage after you started the car.  Customer support at GearVenders told me it had been reported multiple times, but they didn't have a solve, and it drove me nuts until I figured it out.  Easily solved with a relay, because when it drops below coil voltage, it turns off the box completely.  I gave them the details when I figured it out, so I hope they've fixed it by now.

[68stangcjfb]

I have the longer adaptor in my 89' F250 pickup 2wd 5.8 C6, but they make a shorter one for the C6. https://www.gearvendors.com/product/automatic-3-speed-c-6-shorty/ The only fluid I was told to use by Gear Vendors themselves is Red Line 75 W 90 Synthetic and recommended change interval is 5000 miles. That information is from 3 months ago. At 65mph in 3rd with 3.55s and 31-inch tires, I'm at 2600 rpm. In overdrive, it drops to 2000 rpm. My truck is primarily used for towing my open and closed trailers.

[roddster]

  That is exactly what the GV manual says to use....in a truck.

[68stangcjfb]

I just got off the phone with Dave (a tech guy) at Gear vendors. He said you CAN use the Red Line 75w90 synthetic GL 4 Gear lube in a car, but it's not really necessary. Use non synthetic Dextron or a Dex/Merc fluid in cars. DO NOT USE synthetic automatic transmission fluid as that will burn up the clutch because the internal pump produces around 700 psi, and the pump won't make the correct pressure with synthetic ATF. The way I see it, for what that thing costs whether it's in a Pinto with a 4 cylinder or an F 350 with a Power stroke, I'd rather use the best fluid that they recommend which is Red Line 75w90 synthetic GL 4. He also said if you drive your car hard, you should change the fluid more often. If you drive your car like grandma, you can change fluid every 10 to 15 thousand miles.

[charlie D]

I ordered the Gear Vendors overdrive, should arrive around the first of the year. Looks like a good winter project along with a rewire. Will post how the overdrive performs when I'm done.

[pbf777]

     So,..........I was walking around the Daytona Turkey Ron Run yesterday and low and behold there's a vendor with a take-out '68 Triumph TR250 manual transmission with the optional Laycock de Normanville overdrive unit (original "A" series) attached!   

     I said to myself: "Man, that's cool!"; he says: "Man, you need that!"; I said: "Not really, but since I haven't found anything else this cool, not to mention he really was almost giving it away as his wife sent him off to the event with lots of spares out of the garage with instructions of: don't bring them back home!    .........................; now, if I only just owned a TR250!   

     Scott.

     P.S.:  My disjointed relevance to this posting is that, for those unaware, the Gear Vendors unit is a copy (under license) of the Laycock de Normanville unit.