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Discussion Starter #1
Today we collected as much data as we could obtain (and think of) between my truck and gschulte's truck with the hopes of seeing why one truck would get "poor" mpg and another would get "normal" mpg. I get what I consider to be good mileage, he gets what I would consider to be bad mileage. I can get as good of mileage towing 5,000lbs on the freeway as he can get running empty on the freeway.

The big question.........WHY?????? :confused:

Below find 70 mph boost data. Since we had a discrepancy in his speedometer, we standardized the test to RPM of my truck at 70mph which ended up to be 1950rpm. Tire heights were less than 1/4th inch difference so that is a non-factor and the equal rpm means the speeds were very close.

The two spikes in each set are two overpasses on the freeway.

Tell me who's truck is working harder to go 70mph :rolleyes:

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WOW!!!:eek:

Your truck is working alot less to do the same. Are they the same year, mods...?
 

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Are these graphs using the EFI Live? IF not what are you using to collect data?

Bill
 

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Uh, Tx what are we looking at? On the first graph, is your truck the blue line and your friends the red line? Also, what s CFR?
 

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Uh, Tx what are we looking at? On the first graph, is your truck the blue line and your friends the red line? /quote]

What's the UOM across the bottom of the graphs? Also, what s CFR?[
 

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Discussion Starter #7 (Edited)
In all graphs my truck is the blue, gschulte's truck is the red.

First graph is boost, subtract 15 to get actual boost.

Second graph is Calculated Fuel Rate (CFR) in mm.

UOM is seconds.

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Discussion Starter #9 (Edited)
Both trucks are 4x4 trucks and crew cabs, although his truck is a dually. He has the extra rolling resistance of the dual rear wheels, but it looks to be a wash due to my wider 285/75 16 BFG's versus his 235/85 16 tires.

He normally runs his truck set in what we calculated to be a 50hp setting, thats what he calculates his mpg's off of. I detuned my truck to a 50hp setting to match.

One thing we did find mechanically were his front tires were MUCH more difficult to turn than mine.

Holding both front ends off the ground with a pair of forklifts, using maximum strength to physically spin the tires the best you could get out of spinning Greg's tires was 1 maybe 1 1/2 revolutions before the tire would come to a dead stop. On my truck you would get 3 to 3 1/2 revolutions before the tire came to a stop.

Thats a 2:1 ratio at least and likely more in rolling resistance of the front tires. As speed increases would you expect his truck to need more and more power to overcome that extra resistance?

Crawling under his truck I noticed that both of his front axel tube ends showed signs of leakage where the CV joint flange enters the actual axel tube, whereas on my truck they were bone dry. Perhaps his bearings are dry or ruined causing excessive drag?

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One thing we did find mechanically were his front tires were MUCH more difficult to turn than mine.

<snip>

As speed increases would you expect his truck to need more and more power to overcome that extra resistance?
Couldn't you get a ball-park comparison by coasting side-by-side down a hill?

70 MPH @ 1950 RPM?? at 65 i'm right at 2000 RPM 70 I jump to 2150 or so. Why?WHy? Why??
Probably because TxC is riding on 285s.

Joseph
 

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Discussion Starter #13
Couldn't you get a ball-park comparison by coasting side-by-side down a hill?



Probably because TxC is riding on 285s.

Joseph
Thats a great idea, wish we would have thought of that, although we would have probably used flat ground since there were no hills within 50 miles of any appreciable length to try that on.

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Discussion Starter #14
Greg is going to be taking his truck to the dealer for service on those leaking seals, and hopefully will find why the wheels are so difficult to turn.

It might pay 4x4 owners to crawl under their trucks and check for those specific leaks themselves, as well as maybe excessive brake drag. We have had so many discussions of why some trucks get poor mileage, try jacking that thing up and spinning a tire to see if your rolling resistance is in line.

Both of my fronts spun equally as easy, and both of his fronts spun equally as hard, thats why we are thinking he has an issue either in the axel (the leaks says maybe so) or the front brakes are leaving a touch too much pressure on the pads.

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What exactly would we be looking for? Any pics? For the spinning wheel test did you jack just the front end up or the whole truck?
 

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Discussion Starter #16 (Edited)
What exactly would we be looking for? Any pics? For the spinning wheel test did you jack just the front end up or the whole truck?
He had two forklifts, so we lifted the entire front ends of both the trucks off the ground using boards on the forks lifting under the front of the skid plate at the crossmember, right where the two back bolts of the splash shield bolts on. That allowed us to go back and forth between the two trucks so we could readily compare the wheel spinning resistance to be sure what we were seeing was right. We checked repeatedly, with each of us trying the spins and the other marking revolutions using the schrader valve as the marker point.

I didn't have my camera with me at the time, but I just went and snapped the couple of pics below to illustrate where his truck was leaking out of the front axels. It was leaking on both sides in the same place, perhaps high heat from drag creating pressure inside his axels and forcing the lube out?

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Discussion Starter #19
I do want to add that on both trucks you can hear the pads dragging on the discs, but whether his are dragging more than mine we were not able to determine in our limited inspection.

But, just be aware that apparently the brakes drag slightly, at least on 2 out of 2 of trucks checked so far :)

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Tx,
All brake pads will drag, the only mechanism to return the pads is the rotor turning through them. When the pressure is released the tension of the pad on the rotor will push the fluid back towards the brake reservoir. As all of you know there is a height differance from the brake caliper to reservoir. This will leave some tension on the pads. If you had no tension on the pads, the brakes would have a "free feeling" when you first apply pedal.

Most people overlook one of the simplest pieces in the disc brake system, the sliding pins. The entire caliper assembly slides on a set of pins, the same pins you remove when you change pads. These are normally overlooked and will cause excessive brake pad wear, uneven wear, and added friction when they should be free. I replace these whenever I do a brake job if I can, you would be impressed with the differance. By the way, this is one of those things you learn when you start racing stuff hard. In a tough race, I used to use up a set of pads and generally have mushy brakes for several miles, this is one of those little things that make a big differance if it is overlooked. :)

The dually truck will always require more work for the same trip. Two reasons: Weight, he outweighs your truck 645 lbs. stock, that is 10% more weight, more load vs. distance = more work. The second part is aerodynamics, that set of dually wheels are dirtying up the air around the truck way more than you and I can imagine. Dirty air = more drag. The faster we go, the worse it gets. I am amazed at what and where the road grime ends up on the truck when I wash it.
 
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