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Old 04-20-2013, 03:40 PM   #1
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1979 28' Airstream Excella 28
Cedar City , Utah
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Gear Vendors Under/Overdrive

Was going to replace the TH400 transmission in my 1979 Excella Motorhome but now I'm leaning toward rebuilding the transmission and adding Gear Vendors Over/Underdrive.

Any comments pro or con? Anyone else done this?

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Old 04-20-2013, 05:37 PM   #2
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1982 28' Airstream 280
Port Angeles , Washington
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Originally Posted by marydixon555 View Post
Was going to replace the TH400 transmission in my 1979 Excella Motorhome but now I'm leaning toward rebuilding the transmission and adding Gear Vendors Over/Underdrive.

Any comments pro or con? Anyone else done this?
I put one in a couple of years ago and absolutely love it. I have a diesel so it reacts differently than with a gas engine because of the different RPM range but it really makes it nice going down the highway at a comfortable speed with the engine RPM much lower. Actually it is a direct/overdrive rather than an underdrive. The only con is that you should shift out of overdrive on long downhill grades because of the way the gears are cut. It's not a big deal just something to keep in mind.

Cheers, Dan

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Old 04-20-2013, 06:24 PM   #3
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1988 34.5' Airstream 345
The eastern swamps , North Carolina
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Gear Vendors vs 4L80e

Although finally nearing the end stages of a TH400 to 4L80e swap in order to get that overdrive 4th in my '88 345. I would have to say that having a Gear Vendors over/underdrive installed is more expensive than a DIY 4L80e, but, the hassle factor is probably massively less. Also, mechanically simpler, and if you are paying someone to make the changeover, the extra subsystems that have to be changed for the 4L80e could drive the costs through the roof.
As far as real world use goes, I think the 4L80e should have a slight edge if you live and drive mostly in flatter country, and the Gear Vendor should be better in hilly/mountainous country.
Lastly TH475's are immensely strong, and have been around for years and any transmission guy over 40 should have rebuilt plenty of them. And, you do not get into the emergency brake weirdness that goes with a 4L80E.
DO NOT let a transmission chain swap a regular TH400 for the TH 475 that is probably in your Excella now.
Lots of luck with your project .
All the best. Cheers!
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Old 04-20-2013, 07:27 PM   #4
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Have a gearvendor in my '55 Ford tow vehicle. Low (numerical) rear end ratio. 460 cid Lincoln engine. It makes all the difference in the world on those long uphill grades. Also, being able to split the gears gives me (effectively) an overdrive. Boosted gas mileage to 16 from 11. Go for it.
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Old 04-21-2013, 08:38 AM   #5
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1978 31' Sovereign
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Definetly, go with it installed as an underdrive.

Originally Posted by marydixon555 View Post
Was going to replace the TH400 transmission in my 1979 Excella Motorhome but now I'm leaning toward rebuilding the transmission and adding Gear Vendors Over/Underdrive.
I owned a 345 and put many thousands of miles on it. I felt as if the engine was relatively strong but even on rice paddy level ground around Houston I was unable to maintain more than 3 to 5 inches of manifold pressure at 63mph. If I would have decreased the final drive ratio I am sure I would effectivly be at 0 inches of manifold pressure. I do not like to pull engines that hard.

There have been several here in the Forums that have installed the Gear Vendors and none that I have recalled had glowing reports. On the contrary, one gentleman installed one on a 350 and within a week or two the transmission cratered.

Don't get me wrong, I think the Gear Vendor is a great piece of mechanical ingenuity, but I would highly recommend to install it as an underdrive instead of an overdrive. The 454 as installed in trucks and heavily loaded chassis is designed to run at much higher rpm's than most car engines. The "sweet spot" on mine was about 2500 rpm as I recall and with a heavy motorhome the engine could certainly use a few more rpm's.

On most trips I wish I had a few more rpm in the drive train rather than having to run the three speed TH475 in second gear. The original tranny does not have really great cooling capacity and searching for gears and running during torque boost conditions does not improve the life of the tranny.

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Old 04-21-2013, 10:19 AM   #6
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I also have an overdrive and love it. A few years ago, we blew our trans (line from the oil cooler chaffed) and found a old time transmission shop in Idaho. He said he could get me a trans from Salt Lake City, but he would have to rip it apart to re-program it for the diesel. I sorta shrugged, as if to ask, "do I have an option"? He looked at me and said,"I could build you a bad boy if you've got the time." I went with the "bad boy" and have never regretted it. He was able to use a lot of Allison parts, making it the best trans I've ever had!
"A settled wisdom, plus the itch to be elsewhere"
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Old 04-21-2013, 07:06 PM   #7
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1985 32.5' Airstream 325
Lindsay , Ontario
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Dose gear vendors make a unit for the Isuzu with the Allison 4 speed. Dose it bolt up to the Allison.
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Old 04-23-2013, 10:07 AM   #8
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1979 28' Airstream Excella 28
Cedar City , Utah
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A big THANKS to everyone who provided feedback to my message. I am having my original TH400 rebuilt and have ordered the appropriate Gear Vendors direct overdrive. The transmission shop allowed me to purchase the unit myself and thus avoid the 30% markup. They will install it, cut down the drive shaft and re-balance it and install a foot operated switch. The areas where I will typically be driving range from sea level to 12,000 feet so it seems like a good fit. The speed limit here is 75 and even 80 (Nation's Highest) but no one goes that slow so a slow moving vehicle can be at risk.

I anticipate driving 70 mph in overdrive with the RPMs at the same level where they were 53 mph without the overdrive. I hope this means I can cruise comfortably between 62-65 mph without excessive noise, engine heat, engine and transmission wear and hopefully without being run off the road by speeding traffic.
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Old 01-27-2015, 09:46 PM   #9
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1989 34.5' Airstream 345
Waco , Texas
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Mary, how is the Gear Vendors holding up? What's been your experience?
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Old 04-25-2015, 06:40 AM   #10
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I drove Airstream One from SC to FL yesterday. She is just out of storage and had extensive work done over the past months including a transmission rebuild. When the GearVendor unit kicks in all goes quiet.

65 mph at 2600 rpm translates to about 10 mpg in the FC28 weighing in at 13,500 lbs.

Now that she's out of storage, tagged in FL I will begin the process of making her the party bus she wants to be! Let the fun begin!
Glen Coombe AIR #8416
1984 28' Funeral Coach
Former Rolling Showroom & PuttLab (now party bus)
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Old 05-27-2015, 12:01 PM   #11
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1977 24' Argosy 24
Hilden , NRW
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hello marydixon 555
bought last week an argosy 24 feet in USA, and next week is already here in Germany.
I drove on the freeway when I realized that my transmission lacks a gear. I read your comment and I found it interesting that option, you tell me if it works well? that would be a good solution to reduce the RPM and fuel consumption

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