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Discussion Starter #1
A while back I bought a Gear Vendor Overdrive. I've had a love/hate relationship with the thing but I think I have at last made peace with it.

The thing is like having a genius kid that won't do his homework. The MPG improvement seems to be there but I'm had trouble keeping it working long enough to find out.

Mechanically, the GV is bulletproof. Electrically, it's cheap-charlie. It has a control box that automatically downshifts when your road speed gets too low for the hydraulic pump in the gearbox to hold the clutch in - somewhere around 20 MPH. When there is any electrical disruption it defaults you into a disengaged mode.

At first it was the cheesy electrical connections. I'd hit a rough spot in the road and it would drop out. Good quality avionics-grade connections helped but then the brainbox got a wild case of temperature sensitivity. When the outside air temp got below 30 F or above 90 F the darned thing just wouldn't stay engaged.

Apparently this is not a new issue and I should have it worked out. If it stays in OK I'll do some winter MPG tests.

My truck gets about 24 MPG winter in my mixed driving test loop without the GV. One-third state roads (60 MPH), one third Interstate (70 MPH) and one third urban/suburban (moving along with traffic. I like to got about 1500-1600 miles (four fill-ups) so inconsistencies in my filling procedure averages out. In summer I get 26 MPG.

I'm (from earlier partial tests) expecting an additional 1.25-1.5 MPG winter and a full 2 MPG in summer.

From what I've found out about BMEP curves you want to operate in a high load/low RPM mode. Right at the ragged edge of a lug. for me my drivetrain delivers. 1325 RPM and 600 degrees TIT at 70 MPH. 1125 RPM and 550 degrees at 60 MPH.

My aerodynamics for this winter are OK but sub-optimal. I run a flat hard tonneau and a conveyor belt air dam (very rugged). If I had a good aero-lid I'd expect and additional 1.5 MPG. If all goes well, I'll address the aero aspects in the spring.

Let's get a good season of GV operation first.
 

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Looking forward to more of your data.
 

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Discussion Starter #3
After 1350 miles I'm running 24.04 MPG in normal winter driving.

When the cold weather came I had to stop testing and threw in about 5 gallons of kerosene to avoid fuel clouding.

For those not familiar with my test methods, run a fairly consistent routine. One-third state 2-lanes roads, one-third Interstate (70 MPH) and the rest urban/suburban. I pretty much drive the posted limit.

When testing I don't tow. NASA won't loan me the Space Shuttle. Trailers are too much of a variable. I occasionally run bed loads, but mostly I haul my porky carcass - a heroic load in its own right.

I drive about 400 miles between fill-ups and always use at least three fillups to average out inconsistencies in filling.
 

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Discussion Starter #4
So far through 2600 miles in December and January, I've averaged 23.9.

Mixed driving one-third city/suburban, one-third state two-lane roads, one-third Interstate.

Missed one tank because it got under zero and I threw five gallons of kerosene in to keep going without clouding. Winter blend around here sucks.

Crummy conditions reduce speed. Dense air increases aero load. I don't know how much snow I've bulldozed with my air dam.
 

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Dave, I apologize if you have another thread that this question is more relevant..

Have you experimented with smoothing out the underside? Like that stretch plastic you see on boats being shipped on the highway. There would need to be some venting for the exhaust heat etc.
 

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Discussion Starter #6
I've thought of a belly pan, but a lot of the truck maintenance needs access to the bottom.

The air dam obviates the need for a belly pan. The air dam sweeps the air flow to the sides and there is little air movement underneath to generate drag.

At higher speeds you'd need both the air dam and a belly pan. You see this on racing vehicles, but they operate at a much higher Reynolds number.
 

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Dave, how low is your air dam to the ground?

I see that it follows the typical automotive design contour of lower in front of each tire, and higher in the center. I get that blocking air from getting to the front tires has been proven to be a good idea in terms of aerodynamics. I see this repeated in OEM designs of all makes.

But what I don't quite get (without the help of an explanation from you) is why you even bothered to raise the center part of your air dam higher, since it is rubber conveyor belt material that is both flexible enough and rugged enough to withstand being scraped and inadvertent plow duty.

Wouldn't a straight across lower edge have provided you even more of a "sweep" effect, moving air smoothly around and to the side of the vehicle?

Anyways, how high above the ground is the bottom center of your air dam, and how high above the ground is the bottom edge of either end in front of the tires?
 

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Discussion Starter #8
Short answer: 10 inches above grade in center of truck. 7 inches above grade in front of tires.

This is OK for indestructible conveyor belt. Use something rigid at that height and it will get broken.


Testing in abeyance pending some aerodynamic body work.
 
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