I wanted to determine if putting a Gear Vendor Overdrive unit would make since financially so I set up a test to simulate the overdrive unit without actually installing one. I have a G56, 6 speed manual in my 07 Dodge. I determined that if I drove 55 mph in 5th gear and determined the fuel economy and then drove 55 mph in 6th gear over the same course the RPM’s in each of these scenarios would closely simulate me driving 75 mph in 6th gear vs. driving 75 mph in 6th gear overdrive. I determined that the fuel savings would be approximately 17% based on my test runs. I determined that it would take me at least 5 years to payback the cost of the unit. So I decided against purchasing it.

Now the specifics: I decided that I would make the runs approx 40 miles and make them round trip runs. I live near highway 99 in Northern California so decided to start at Arno Road and go thru the towns of Galt and Lodi and turn around at Hammer Lane near Stockton. I ran the test during the day when traffic was light and I could maintain a steady 55 mph using cruise control. I didn’t want to fill up at a filling station because filling stations are on more elaborate freeway exchanges and would require stopping at stop lights etc also I didn’t want to use a filling station because I wanted to be more accurate with the fill. I made an acrylic clear cylinder about 7 inches in diameter and about 30 inches long and accurately made calibration marks so I could determine fueling to the 1/100th of a gallon. I mounted the cylinder on my camper directly above the fuel fill pipe and took it off while driving. (See picture) I would fill the cylinder nearly to the top and then write down the quantity. I have a valve on the bottom of the cylinder with a valve and I would fill the tank to the brim till just before it started to drip out the filler spout. I had to rock the truck a few times to insure the fuel stayed at the filler spout adding a bit more fuel each time I rocked the truck. I then noted the fuel remaining in the cylinder to determine the total fuel used. Of course I started with a full tank on each run. A month before I did these runs I tried to do the same thing but found it very difficult to completely fill the tank so got on the Diesel Truck Resource and found what my problem was. Dodge mounted the tank vent line below the high point of the tank so there was a cavity above this vent line in the tank that filled very slowly. There is a very small vent at the top of this cavity so if you tried for a long time you could fill the tank entirely but I wasn’t patient enough to wait this long.. I have a camper mounted on my flat bed that weighs 3000lbs so fuel economy is less than an unloaded rig. I made a total of 5 runs, three in 5th gear and two in 6th gear. I figured for the test to be valid it needed to be repeatable. (And, as you will see, it was) I started at 10am and finished at 2pm, the wind was very slight, the temp was 55 degrees, the sky was mostly cloudy, the ride rites were inflated to 30 psi, the RPM was 2220 in 5th gear, 1750 RPM in 6th gear, the distance of the round trip was 44.6 miles and the A/C was off. The following are the three results when running in 5th gear: Overhead MPG indicator: 16.2, 16.6 and 16.8. The fuel used was: 2.39 gal, 2.41 gal and 2.42 gal. The following are the results when running in 6th gear: Overhead MPG indicator: 19.6 and 19.7. The fuel used was: 2.07 Gal and 2.04 Gal. In 6th gear I got an average MPG of 21.6 and in 5th gear I got an average MPG of 18.5.

I personally think the best way to go is to change the ratio in my rear end. I currently have a 3.73 gear set. The highest ratio that is made is 3.43. I talked to U.S. Gear today and they said they would make a 3.20 gear set that costs approx $450 if I had 100 people sign up. Anyone interested? If a person towed heavy this probably wouldn’t make since but with my G56 transmission my first gear is very low and I never use it (I just haul my camper and tow my daughter’s 24 foot trailer occasionally) My truck is two wheel drive.

 

I think the only time it makes since is if you haul heavy half the time, and empty the other half. Then you'de have the perfect gearing for both situations.

 

John,

I'm impressed with your test! I'm also surprised at a couple of things.

Your signature says 3.73 gears and your note says .79 OD ratio. I can't remember right now the 48RE OD ratio, but I'm running 1500 RPM at 60 MPH calibrated for 285/75 tires with 3.73 gears. It seems you're spinning quite a bit faster.

I agree that to get a higher ratio you'd be better off changing the differential gears than adding a gear venders unit. Mainly because the extra set of gears would cost, probably, 5% or so of your power and might cost you any mileage savings in added drag. An even better situation would be to get a straight through tranny (non OD) and go to about a 2.5 rear end. But I don't think that's practical or even available. Higher gears through OD has always seemed like a gimmick to me.

Meanwhile, maybe that was you I passed, up by Lodi, the other day. Next time wave or something!

 

i have a question, did you hand calculate the averages? i know that the overhead is no way an accurate means to measure the mileage. but great test! i still plan to get one. i'd like to curise down the highway at 75 and still be able to talk w/ someone in the truck...whether i'm towing or not.

 

If you never tow I would definitely swap rear end gears. It will be cheaper than the GV. The .79 ratio and 3.73 's work good for towing but empty leave a little mpgs. Less hassle to do the rear end swap. Less parasitic loss from aux tranny.

 

When I bought my truck the 3.42 gears weren’t available so I bought a GV unit to complement my 3.73s.


3.73s would be 55 MPH at 1750 RPM in 6th
3.42s would be 61 MPH at 1750 RPM, or ~1600 RPM at ~55 MPH.
3.20s would be 65 MPH at 1750 RPM, or ~1500 RPM at ~55 MPH.

The GV engaged in 6th gives me 71 MPH at 1750 RPM or ~1340 at ~55 MPH. (RPMs too low).

Sounds like with your towing, and the fact you have a 2wd, new gears would be the best bet, if you can find them.

I have 3.42s (In a box) for the front, but the rear gears are on back order. Long term I want to retire the GV.

 

mitternocht, If you don't mind me asking, why do you want to retire the GV?

 

I never liked the slip yoke on the back of the thing, it is only 3” long and uses a 1350 U-joint, our stocker uses a 1485 and is twice as long. Is this an issue? Probably not, but I don’t like the design.

When you have a 4x4 it hangs off the back of the transfer case, far from the transmission mount. I don’t care for this cantilever type of design. When engaged it throws a torque back to through the transfer case up to the engine mounts. Even when not engaged I think this cantilever exacerbates the “hop” issue these trucks have. I tend to drive off-road and in the snow now and then and don’t want the hop.

I don’t care for the “be careful in reverse” issue, ether. If I need to pull something or someone in reverse, I don’t want to worry about it.

I think the 28.6% OD the GV provides is too much for our G56s Trans. It really only gives you “7th gear” and “3rd and a half”. 5th over is the same as 6th, 4th over is just below 5th, 3rd over is a pretty good split, however you have to make sure you are over 20 MPH.

Don’t get me wrong, the GV works really well and gives a perfect “7th gear” ratio for me, and I would have bought it again. However, now that 3.42s are out, and the newer G56 has better gearing, I can almost match the GV gearing with those and some larger tires. I’ll either sell it or get the parts so I can adapt it to the back of a 727 for a muscle car project I have. (Not retired, reincarnated!! lol)

 

I was interested in one. I contacted GV about towing in 6th over and they said it was not recommended and the unit would not hold up towing in 6th over. The tech said something about driveline vibration being the issue.

 

Thanks mittnocht for the reply. I'm thinking of trying one in the spring and have never heard of many people that wanted to get rid of theirs. I have also been concerned about how far back it would be mounted without any additional support (the info on the U-joints is also much appreciated). I do love my AD G56 for towing but need the 7th gear for unloaded freeway driving (which I do often). I think I will still try a GV and I'm hoping to see the mileage improvement/better driving experience that some have reported.

 

Answering questions posted: Raspy: Gear Vendors claims their unit doesn't reduce the effeciency of the drive train by much. They claim you will use one extra gallon of fuel for every 400 gallons you burn. nwflyoda: You asked if I hand calculated my findings. Yes, the overhead reading info was just as a comparison.
If I changed to 3.42 gears, does anyone know the cost of the gears and is it something a home mechanic could do?

 

The front gear set cost me ~$300, however I still need to order some pinion bearings, seal, etc. (it came with pinion bolts). The rear gear set is ~$400, however I didn’t get it yet so I’m not sure that is in the kit, although I think it comes with bearings, seal, crush washer, spacer...etc.

Can a home mechanic swap gears? Yes. Will he do it right and will they last? That is to be determined!! I bought a pinion depth gauge, some gear marking compound.... and already have a magnetic stand with a dial micrometer......need to fab or buy a pinion wrench....

Been reading how it is done, but I can’t say I’m not nervous.

 

I had to tear down my rear diff. when I was 1200 miles from home last year. The best suggestions that I have for you are to entice a friend come and help because the stuff is heavy and read the instructions and get all of the needed tools ready before you tear into it.