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Old 04-10-2017, 01:04 PM   #1
nm78bronco
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Overdrive for a 1976 GMC C15 suburban with a 454

Hello All,

I'm really interested in getting an overdrive for my 1976 GMC C15 suburban with a 454 engine.

Currently, I have a TH400, 12-bolt with a 3.07 ratio. I recently purchased a NOS 3.73 12 bolts gears on ebay for installation.

My truck will be primarily for day trips and around town (no towing), however, I plan to tow up to an 8000 lb trailer, up to 10 times per year.

The 2 options I'm considering are:
1) gear vendors overdrive
2) 700R4 OD

My dilemma about the gear vendors is I keep hearing about unwanted drive train vibrations which I want to completely avoid. Does anyone know of an excellent gear vendors installer in Arizona, who successfully installed a gear vendors in their truck with no vibrations? I really want the the truck to be vibration-free like the factory.

From what I've read, the 700R4 can be built to do what I want if driven intelligently. It lacks the gear splitting of the gear vendors which is a major minus, however, again, no vibrations is very important to me since the vehicle will be doing alot of highway driving. I'd need to install a huge auxillary transmission cooler for this treanny. Did the factory have auxillary transmission coolers for the tow package trucks?

Thanks for any comments
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Old 04-10-2017, 02:21 PM   #2
68Timber
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Re: Overdrive for a 1976 GMC C15 suburban with a 454

I guess it's fair to say the R4 can be set up to do that job and may be OK doing it. Me, I just wouldn't try it. Big truck, big block, 8K pound tow, hot climate? etc. I'd contact GV and ask them to recommend an installer in your area.
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Old 04-12-2017, 11:23 AM   #3
nm78bronco
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Re: Overdrive for a 1976 GMC C15 suburban with a 454

Thanks alot for your input. The camper I plan to buy will probably be ~3000lbs to 6000 lbs, I'm not sure at this point. Loaded it would be up to 8000lbs.

Is there anyone who has a gear vendors installed on their 1/2 ton 2WD suburban with excellent results (meaning no driveline vibrations)?
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Old 04-12-2017, 12:28 PM   #4
68Timber
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Re: Overdrive for a 1976 GMC C15 suburban with a 454

Forgot to mention, and you may have seen the ads, GV advertises that their product is very popular among the Drag Week participants. They've been around since I was a kid, if vibration was an inherent problem I'd like to think they'd have disappeared from market a long time ago. I'm not dismissing those who have a vibration problem, but I'd suspect the install and not the product. Jmho.
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Old 04-13-2017, 07:48 PM   #5
nm78bronco
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Re: Overdrive for a 1976 GMC C15 suburban with a 454

I called GV today and I think its the way to go. You're right. Their overdrive would not sell if it was problematic.

Living in Arizona, when you goto the mountains, the gear splitting sounds like an excellent option, especially when towing.
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Old 04-15-2017, 01:19 PM   #6
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Re: Overdrive for a 1976 GMC C15 suburban with a 454

I think you'll love it. I would. GV behind a TH400 is as bullet-proof as you can get. Add a big aftermarket trans cooler and you'll never have to worry about it again.
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Old 04-16-2017, 09:20 PM   #7
nm78bronco
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Re: Overdrive for a 1976 GMC C15 suburban with a 454

Thanks for all your comments. I really appreciate your time.

I need to figure out where to mount an auxillary transmission cooler on my suburban. My suburban already has an engine oil cooler in front of the AC condensor on one side. If I put a large transmission cooler on the other side of the condensor, I'm wondering if that will hurt my air-conditioning performance. A hot day here in Phoenix is 114F!! Anyone have suggestions of transmission coolers mounted in other locations than in front of the ac condensor?
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Old 04-17-2017, 01:26 AM   #8
SuburbanStevie
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Re: Overdrive for a 1976 GMC C15 suburban with a 454

I've got a 1990 Suburban R2500, 454, 3.73. I had a GV installed about 20 years ago. And I do occasionally tow a trailer that weights around 7500 lbs.

Do I like it? Yes. Why?... Not for the fuel economy, as I would still be ahead had I not bought it. The gear-splitting is great for trailer towing in mountains, and I like the sound of lowered RPM at freeway speed.

Vibration?... Maybe a tiny bit above 80 mph. Obviously GV will only distribute through trans shops that know what their doing. But I'd go and chat with the shop manager and make sure he knows what he's doing

Your vehicle is a bit older than mine. Are you sure you want to sink that much money into it?

PS. The only service it has seen is when I've changed the oil as required.
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Old 04-17-2017, 07:53 AM   #9
Edahall
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Re: Overdrive for a 1976 GMC C15 suburban with a 454

One big advantage of the GV is the ability to tow in overdrive. It is more than strong enough for this while the 700R4 is very weak in this aspect.

Why are you going from 3.07's to 3.73's? The 3.07's with your combo should work out pretty good.
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-3.73 gears -2" Lift

Last edited by Edahall; 04-17-2017 at 08:00 AM.
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Old 04-17-2017, 12:24 PM   #10
phillipm
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Re: Overdrive for a 1976 GMC C15 suburban with a 454

Just need to re check your pinion and driveshaft angles after install. Now that your power train package is longer it changes the angles. That is where the vibes come from. Reset those with some shims and you should be good to go.
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Old 04-17-2017, 12:35 PM   #11
nm78bronco
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Re: Overdrive for a 1976 GMC C15 suburban with a 454

I'm interested in the gear vendors primarily for gear splitting in the mountains.

Living in the Phoenix area, when you leave town and go to towards the mountains, you are immediately dealing with mountains for extended driving.
I do plan on towing several times per year. The GV will cost alot and so I am thinking I will first see how much the trailer will weigh that I buy. (probably 4000-5500lbs unloaded). I'll see how my 3.07 gears handle that trailer with no gear vendors. Then I will decide whether to buy the gear vendors or not. Since I live so close to the gear vendors site, I would drive a little ways to have them install it for optimal results if I decide to buy it.

Another factor for the gear vendors is just the cool factor. The 3.07 ratio on my suburban really tames the 454. With a 3.73 and a gear vendors, my OD ratio would be 2.91 instead of 3.07 (not much improvement), however vehicle takeoff should be better as well as compression braking in the mountains, which would assist my 1/2 ton factory brake system and therefore, safety.

Thanks for all your comments and questions. I really appreciate them.
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Old 04-17-2017, 03:35 PM   #12
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Re: Overdrive for a 1976 GMC C15 suburban with a 454

4.10s and the GV is what I have in my flatbed. 1 ton, 10' flatbed dually. At one time I was trying to slow the engine down and had 3.73s. What a waste of money. I put in 4.10 and it made all the difference. Only a 10% difference, but it was all that was needed. 22 percent overdrive with the 4.10 puts you at a 3.19 final which will still tame the big block, works for me anyways
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