cwgtex

I would like to share with everyone my experience with my latest toy. I bought a Gear Vendors auxiliary overdrive (http://www.gearvendors.com). It is basically a small second transmission bolted behind the transfer case. There is a lot more info on their website about it, but I mainly bought it to lower my highway cruising rpms. Here are some installed pics.

Side view.


Front view.


Adapter between unit and transfer case.


Shortened driveshaft.


Rear view.


In cab controls.


Master switch.


Shifter-mounted activator switch.


Once the I hit the switch, the red light (auto-drive) lights up to let you know that a shift is commanded, and then it switches to the green light (over-drive) once the unit successfully shifts. This process only takes a second. However, if you are going less than 18mph, then it won't shift and the red light will just stay lit until you get over 18mph. You don't have to hit the clutch to shift, although its pretty rough if you don't.

Right now I think the inside controls look a little tacky, so I will be redoing it soon to hide the wires better. As far as use, 4th over is pretty close to 5th (1.08 vs 1.00) and 5th over is even closer to 6th (.78 vs .79). Because of this, I did a little ratio math, and I've decided the best way to shift for me is 2nd, 3rd, 4th, 4th over, 5th over, 6th over. There is no issue going from one gear to the next leaving the GV engaged (say 4th over to 5th over). Using this pattern gives me the following shift percentages compared to factory:

How I shifted before (G56)
2nd 3.48 > 40% shift > 3rd 2.10 > 34% shift > 4th 1.38 > 28% shift > 5th 1.00 > 21% shift > 6th 0.79

How I shift now (G56 w/ GV)
2nd 3.48 > 40% shift > 3rd 2.10 > 34% shift > 4th 1.38 > 22% shift > 4th over 1.08 > 28% shift > 5th over 0.78 > 21% shift > 6th over 0.62

My RPMs at 70 dropped from 2100 to 1600. In fact, my truck lugs a bit at 1600 so I don't put it in 6th over unless I'm going 75 or more. Once my speed falls below 75, the EGTs start climbing (100-200 degrees). Hopefully when I upgrade my turbo it will take care of the EGTs and I'll have more power at 1600 RPMs so I can cruise there at 65-70 mph.

JFlory

Great write up with pics. This is something I would love to add on my brothers tow rig.('01, 5-speed)

What kind of fluid does the GV take, and does it need changed often? Any filter internally? How long did the install take and what special parts were needed?

I know, lots of questions, but I am serious about installing one in the next few months. Right now i've been pricing them out and waiting for my brother to pull the trigger. What you have looks great and the ratios are spot on for what I was looking for. Thanks.

rip 112

Man I want one!!!! Great job with the pics and stuff! Post a vid if you can of it in action.

dslpwr81

What have you seen as far as a mileage diff?

mitternocht

Nice write up. I run Amsoil in mine and change it more often than it calls for. I’ve never taken mine apart for cleaning, but it is probably due. I’ve put about 20k on it.

It has a safety interlock so it won’t work when you are in 4 wheel drive, however if you use 4x4 then shift to 2 wheel drive the GV won’t engage until you key off the ignition. Good for safety, but annoying when coming off the trail

It does shift off at 18 mph, so you have to not forget to clutch or it will shift for you. (Not good).

Sometimes if you coast to a stop in neutral or use the clutch and forget to turn it off it will auto re-engage at 22 MPH. This isn’t expected and doesn’t happen all the time. Sometimes it won’t engage until it “warms up a bit”. I’m not sure what is up with this, maybe it needs cleaned.

It’s hydraulics running from input RPMs so it also shifts faster with speed. In other words if you catch 3rd over at 25 you need to hold the clutch in a bit longer than catching 6th over at 65.

I can grab 6th over at 60 with no issues, the smarty helps with low end torque, however you can’t roll into it in 6th over at 60 or she’ll roll the coal. After running for a while empty I started to see 2k as “high RPMs”, lol. I have stock tires, however. Those 315s make a difference.

It is a good unit and I’d buy it again, however I’d like to see what the 3.42 gears and slightly larger tires would be like. It sure is cool with those extra gears, however.

Again, thanks for the nice write up with pixs, I’m sure others will appreciate them.

georgboy

How much does the unit cost? How much did the whole install cost total?

tryNto

I would love to have 1 of these, but the 3 grand plus is a little more then my budget for awhile......

dslpwr81

I priced one out for my 06 a month ago and it was $3390 to the door. I heard its a pain to install on an auto, but I want it. A guy I know has one on his auto and likes it. Seems a little strange to me to have one connected to an auto, but....

Nsomniac05

Where did you get the rear drive shaft. And what are the specs on it?

cwgtex

I haven't changed the fluid yet, but the interval is 5k miles. The manual has some kind of oil listed; I am planning on calling Amsoil and ordering their equivalent.

I am unsure about the filter, I need to read through the rest of the manual.

I was lazy and paid an RV depot to install this for me. It took them 6 hours to install.

Honestly I'm a little disappointed with the mileage improvement. Before I could get about 20 mpg going 60. In 6th over at 60mph, the RPMs are too low (1500 I think) and I'm lugging it. However, the best RPM for mileage is about 1800, and I do that in 6th over at 80mph. Now I can get the same 20mpg at 80mph , but no better. At that speed the wind resistance is just too much.

My thinking is that if I had smaller tires (closer to stock) I could sit at around 1700 at 65 and get the best mileage, probably 22+. As it is now, I am happy getting 20 at 80mph.

When I upgrade my turbo (Garrett 3788 stg 3) hopefully I will have a lot more air and power at 1600 RPMs and then maybe I will be able to cruise at 1600 RPMS and 65 mph and get 23 mpg (just guessing).

Too dang much!

Here is the breakdown:

$2590 gear vendors
$75 shipping
$540 labor
$150 driveshaft work
$265 tax
------------------------
$3620 total


I got the driveshaft from between my transfer case and my rear differential .

I'm just messing with ya, but seriously, they just shortened my existing driveshaft, I'm not sure exactly how much.

Nsomniac05

clever. 2 caps and needle bearings are gone on my rear U-joint, and im using it as an excuse to get a gearvendors and A new driveshaft, hopefully aluminum.

cwgtex

There you go, save some weight.

Ace

I put a GV on the '91 about two and a half years ago. Best mod ever. It runs 1700rpm at 65mph in 5th over with 4.10s and 34" tires approaching 23mpg steady on the interstate at that speed. Still gets 21+ running 70mph. In-town mileage suffers with those tires, though. Oh well, everything's a compromise.

I extended my drain intervals by installing an external sump w/filter, increasing the fluid capacity to about 5 quarts:
http://stuff.is-a-geek.net/PhotoAlbu...cs/CTD_125.jpg
http://stuff.is-a-geek.net/PhotoAlbu...cs/CTD_126.jpg

cwgtex

How difficult was this mod? How expensive was it? I am interested in this. How often do you change the oil now?

Ace

It was fairly easy and inexpensive by my estimate. Mostly stuff from the big auction site - Mack truck steering resevoir/filter($30), old style Autopulse pump ($25), maybe $30 worth of tubing, clamps, barbs, bolts, etc. plus some scrap steel for custom-made brackets to bolt the stuff on with. You can't buy those, gotta make it yourself. I installed it after the first year and oil change at about 6k miles on the first fill, IIRC. It's got 18 months and around 15k miles on this sump. I'm going to let it go 2 years and see what it looks like. Maybe longer next time, we'll see...

I run the pump once or twice a week for just a few minutes at a time on the way home before pulling into the garage to keep fresh, clean oil in the GV.