I was reading up on the gear vendors overdrives and they said the following:

Fuel Economy: Besides giving you 8 gears and lots of performance the Gear Vendors Under/Overdrive is the most effective way to improve fuel mileage on vehicles running a 3.73 or higher numerical rear gear (ie 4.10, 4.88). Expect 20% increases during highway cruising with any rear gear in the 4's and 10-15% with a 3.73 with any big block engine gas or diesel. Small block V8 engines (4.6, 5.0, 5.4) do not have the bottom end torque to push the trucks aerodynamics in double overdrive so on the small blocks your factory gear should be good (with the exception of the supercharged small blocks).


Does anyone on here have an overdrive on a 351W with a C6? does it have enough torque for the highway?

Edit: just realized they are talking about double overdrive, like for a A4OD in 4th with the the extra overdrive on top. So i assume it would be fine for a C6 then.

 

Yeah, I don't have any experience w/ their overdrive, but I have a truck similar to the truck in your sig,, & the c6 definitely seems like it could use an overdrive. When you're in high gear by 30MPH, you're always looking for another gear.

 

I don't now about their setup, but a regular overdrive transmission works fine with our trucks. Why not just go that route?

 

If I'm reading the Gear Vendors information correctly, they seem to be talking about adding their overdrive unit to a transmission that already has overdrive (4R70 W, E4OD, etc.). With that situation, you will be getting a 0.7 to 1 overdrive in the existing transmission, multiplied by the 0.8 ish of the GV unit, for a 0.56 to 1 overdrive ratio with both the transmission and overdrive unit working. So, with a 4.10 rear end gear ratio, with just the 0.7 to 1 overdrive you would be at a 2.87 to 1 effective rear gear ratio, while with both the transmission and the GV unit engaged you would have an effective rear gear ratio of around 2.3 to 1. Basically, if you were running at 4100 RPM in drive at a certain speed, you would be running 2870 RPM with just the transmission in overdrive and the GV unit off, and 2300 RPM with both the transmission in overdrive and the GV unit ading additional overdrive. Hopefully, this clears up the gearing and RPM part of the discussion.

As for their claims on engine torque output with the engines mentioned, I would be leary of adding the GV unit to an overdrive automatic with those engines UNLESS you had rather high numerical rear end gears (4.88, 5.13, etc.). Then I would still go through a gearing calculator to see what my RPM at various speeds would be.

NOW, getting back to the vehicle in question with the C6 automatic, I would look into a Gear Vendors unit for that vehicle if you are doing a significant amount of highway driving that would justify the price of installation vs. the improvement in gas mileage. Again, I'll leave this decision up to you as to whether or not it is cost effective to add the GV unit.

 

I don't see a Gear Vendors OD unit producing the milage improvement they claim in these trucks specifically because of the poor aerodynamics they have. High drag at highway speeds combined with a super deep overdrive ratio will put the engine under heavy load and that triggers open loop operation with EFI so fuel consumption goes up not down. The GV unit is a good addition to a truck that doesn't have an OD trans but most trucks that have an OD trans won't benefit from it, and even then you could probably produce the same milage improvements for less $$ with a relatively cheap gear change or an OD trans swap.

As for the title of this thread, I'd agree the 4.6 and 5.0 motors don't seem to produce the TQ to be able to maintain highway speeds in the bigger trucks on anything but flat roads particularly when they are under geared.. as most of them are. These smaller V8's need 3.73's or more depending upon tire size just to handle a factory overdrive transmission so adding an additional OD ratio isn't really an option. The 5.4 and 5.8 on the other hand wear quite a bit more hair on their chests and can pull taller gears.. but I still don't see a 0.5:1 overdrive ratio getting much use on anything but ruler flat highways.

 

Cheers guys