Over / Under drive units ? Anybody running one of these ?

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87BlazeLS

Master Mechanic
Sep 5, 2012
402
30
18
Apopka Florida
Hey friends, Now I'm not talking about blower drives here. Just the over under drive units that attach to the transmission that alters the final drive ratio.

If you are running one of these please comment on the good and the bad of having one and try to include
details on make and model of the unit and the transmission / car it was in. Thanks
 

Texas82GP

Just-a-worm
Apr 3, 2015
7,981
18,676
113
Spring, Texas
I bought a gear vendors overdrive for my dad for his 62 Ford Galaxie. His car has a Borg Warner T10 four speed. When I bought the 'kit' I was told it was a drop in install and all I had to do was put a shorter output shaft from a Mustang in the T10. I found and bought the shorter output shaft, Dad and I pulled the T10, tore it down and swapped in the shorter shaft and installed the Gear Vendors tail housing. We then went to put the trans back in the car. There was no way it was going to work. Both the shifter and the trans mount were totally in the wrong spot. I called Gear Vendors and they said 'sorry, we were wrong. You have to swap a Toploader transmission in the car to work with our overdrive." We pulled the T10, tore it down, put it's original output shaft back in it and put the car together. We shipped some of the Gear Vendors 'kit' back (tailhousing adapter, etc.) and they sent their stuff (tailhousing, shifter brackt, etc.) to work with a toploader. I find and buy a toploader for the car in the meantime and rebuild it. I get the new tailhousing adapter from Gear Vendors only to find the toploader output shaft is too long. I call Gear Vendors and they say "we meant to tell you that you will need to put the Mustang length output shaft in that toploader for it to work." At this point I'm pretty frustrated. To Gear Vendors credit they tried to make it right and sent me a new output shaft they sourced from a vendor. So dad and I tear down the toploader and install the shorter output shaft, tailhousing adapter, pull the T10 out of the car and install the toploader only to find the gear vendors overdrive unit won't fit because of dad's stock style h-pipe in his exhaust. We also discover that while the shifter is in the right spot, their bracket has it needlessly angled way forward which won't work with the stock console and the transmission mount pad is way too short so a very thick spacer will have to be fabricated to correct the driveline angle. So the next thing we did was to put the T10 back in the car and drive the car to an exhaust shop to have dad's stainless h-pipe exhaust system modified into true duals. At this point I think we can get it in but it will require trimming of a floor brace, the spacer for the trans mount I mentioned, some engineering on the shifter, shortening of the driveshaft (we knew about) and who knows what else. Long story short it was not a drop in as advertised, rather it's a totally custom install. Gear Vendors will tell you what they have to in order to get the unit sold. I spent around $4K trying to do something nice for my dad and it has been a nightmare. On a car like dad's, it's about the best way to get overdrive in it. On something like a g-body, save money and headache and do a transmission swap. I should have done more research before buying the Gear Vendors. Lots of people online complain of vibrations from bad driveline angle after the install (numerous different applications, both manual and automatic). If you have the talent and the persistence the unit can be installed but it is not a bolt in deal. Sorry I can't tell you what it's like driving it yet but wanted to share my experience in case you are thinking of going that route.
 
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87BlazeLS

Master Mechanic
Sep 5, 2012
402
30
18
Apopka Florida
Thanks tex82, I kind of figured some issues with fitment on some vehicles.
Drive line angle too can cause vibrations that are after the fact. And only discovered
after the mod is done or on a test drive or dyno.

I see your GP used to be a diesel. The 350 version?

I had a 79 chevy pickup with one, turbo 350 trans. Diesel drove ok for about a year.
Had a pretty good oil leak too. Then started belching smoke rings out the dual exhaust.
Yup blown head gasket. The gaskets were cheap, to change them out was not even
on my budget radar. I ended up buying a 71 Delta 98 with a bad rear end but a good rocket 350 and installing it.
Great truck after that.
 
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vanrah

G-Body Guru
Apr 16, 2013
879
1,189
93
Near Afton, Wisconsin
Greetings Guys; I'm sure glad you started this post 87BlazeLs as this has been on my to do list since 1997. But I went the cheep route with a 700R4 in 98 only to destroy it by the end of summer & 4400 miles. I also didn't care for the lower first gear ratio of the 700R4 unit. Yes I know that tech has made great inroads into the life cycle of the 700 in recent years, but they still have a lower first gear than my TH350 unit. And I've had great service form the TH350 as I've only had to rebuild it once in the 36 years I've owned this automobile, around 90.000. Plus it shifts every time I hammer it right at 6400-6500 RPM and the 700 would not do that repeatedly (?). So a Gear Vendors is back on my to do list maybe 2 or 3 years out unless I sell it (Bronze Brick) first. Who can tell the future, I lost my crystal ball & the last time I seen it, it was all cloudy.
Texas82GP, if I can cheer ya up. If having a nice car with zoot was easy everybody would have one. And of course hind sight is perfect, maybe a manual 5 or 6 speed would of suited your dad better. I sure hope if (when) I get to that point again that the mating up to a TH350 would go much easer??? Thanks for the read, Bob Jr.
 

87BlazeLS

Master Mechanic
Sep 5, 2012
402
30
18
Apopka Florida
It would seem from texas82 response earlier that much of the issue was them not telling him all the required
details until the ut oh. Seems they should offer some sort of fitment paperwork for installation. Even if
it was some what generic in nature. Also in his case (an early ford) I think I would have tried to get the
car up on a lift and takes some photos of the chassis and exhaust just to see what would maybe need to be
relocated or altered for a better outcome. Several things happened to make his experience go bad.
In the many car shows I've attended over the years I've only seen one example of a nice muscle car
w/ a gear venders overdrive. And that was a perfect example 67 Impala w/ 427 and 3 speed floor shift.
Very uncommon combo. His over drive worked just like the forth gear. No trailer queen either. He drove it all the time.
 

565bbchevy

Geezer
Aug 8, 2011
9,591
12,612
113
Michigan
My buddy has a 69' Nova with a 598 BBC and a GearVendors, he bought the car with it about 6 years ago and he drives about an hour on the freeway to come down to my house so we can hit the local car shows in the summer time.
He gets decent mileage with it also.
I have been wanting to get one but I would have to redo my exhaust since the balance tube would be in the way and I am not sure there would be a spot for it with the GearVendors installed.
I have also considered looking for a built overdrive trans to solve my exhaust issue but ones that can handle my power cost more than the Gear Vendors and still would probably not be as strong.
 

87BlazeLS

Master Mechanic
Sep 5, 2012
402
30
18
Apopka Florida
That makes sense since your running large *ss horsepower. You are in the hunt for high end parts now
as high as you are on the build table. Anything less will result in breakage.
But your ride is more than awesome....
 

thepcsurgeon

Master Mechanic
Oct 10, 2013
252
26
28
Orange County, NY
Well, I have one installed on the Monte. Had it now two years and LOVE the thing. What do you want to know? I have a TH400 manual reverse VB and all I really needed to do was make a slight dent in the floor to make a little space. That's All.
I cheated!!! Found one used (TH400 model) about 100 miles from me. Got it for $800. Installed it and blew it up ( knew it was old but the price was fantastic). Once I killed the clutches in it, called GV and they shipped out a rebuilt unit for $900 plus a $500 core charge which was credited back in 5 days after I sent the core back. Saved $1000 this way.
The unit drops 700RPM's when engaged. Currently I'm running a 3.50 rear gear in a 9" found housing. 75MPH makes 2300RPM on the highway. Easy to install and well worth the money for me. The gear splitting is not for me, I strictly use it as an overdrive.
 
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Turbo Zach

Comic Book Super Hero
Sep 8, 2015
2,534
5,874
113
50130
I have thought about getting one for my dad's Nova. I'm afraid that the 950 holly would load too bad. I'm very interested in this thread.
 

87BlazeLS

Master Mechanic
Sep 5, 2012
402
30
18
Apopka Florida
Sounds like pcsurgeon is the man in know on this one. So any details he has on it would be great.
Post some pics of that set up. PLEASE !

So I wonder how well it would work as an under drive ? Very interesting commandant...
 
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