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Old 09-09-2016, 10:37 PM   #1 (permalink)
Captain Slow
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Which gear for max. MPG at 60 MPH while towing?

The purpose of this posting is to stir conversation and to pass along useful information. Please add some data to your comments if you can, it most likely will motivate others to agree or disagree.

Hypothesis
Which gear in a Silverado 2500HD with an Allison 1000 transmission at 60 MPH results in the maximum MPG while pulling a 9,000 travel trailer on level ground with a few highway bridges?

Data gathering
Information was gathered with a Banks IQ OBDII interface. Each of the data groups was gathered over a 10 miles stretch of highway at 60 MPH while pulling the travel trailer. The interface had a manual reset, which was used to start the average MPG calculations at the beginning of each test run. Cruise control was engaged during each run.

Allison 1000
ALLISON 1000 RATIOS

1st------------2nd----------3rd----------4th----------5th----------6th*----------R
(3.094 : 1)--(1.809 : 1)--(1.406 : 1)--(1.00 : 1)--(0.711 : 1)--(0.614 : 1)--(4.480 : 1)

Note: The two gears involved in this test are 5th & 6th gear. Also take note that these two gears are both overdrive! When running in 5th gear vs. 6th gear the RPMs increase by approximately 250 RPM to about 1700 RPM at 60 MPH. The LBZ Duramax Peak Torque specification is 650 lb-ft @ 1600 RPM

Original design strategy by General Motors
My personal assumption is that GM optimized 6th gear (second overdrive) for maximum MPG for a truck that spends most of its life with no load or some load in the bed for the majority of the truck owners. The minority of the owners who really use their truck to pull heavy loads are not given much thought in the finished design concerning MPG. Obviously the “Tow-Haul mode” is a big plus for heavy hauling but the transmission still tops out in 6th gear.

Speed chosen for test
I realize that 60 MPH is too slow for most RVers but it worked out well because my truck runs well at that speed in either 5th or 6th gear. Also when doing the test on the interstate I could run for 10 miles in the right lane and not have to change lanes to avoid running up behind someone or slow down and void the test.



Test data for Average MPG @ 60 MPH

Test-------5th----------6th---------------5th gear MPG delta

#1--------12.9---------12.3--------------.6 MPG
#2--------13.6---------12.9--------------.7 MPG
#3--------13.4---------12.9--------------.5 MPG
#4--------13.8---------13.3--------------.5 MPG
#5--------13.8---------13.2--------------.6 MPG

Results (Average of the five test groups)

At 60 MPH the test results show an improvement of 0.58 MPG. My conclusion is the added resistance of pulling a heavy load in the second overdrive keeps the engine from working in the most efficient range of the torque curve.

This data indicates the stock performance of my truck while pulling a heavy load does not result in the best MPG. If I choose I can operate my truck at a different point on the torque curve to improve the MPG by using 5th gear.

I know most everyone has thoughts on this subject so please leave your comments. My data has a sizable margin of error I am sure, but I think it does show a trend.

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Last edited by Captain Slow; 09-09-2016 at 10:56 PM.
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Old 09-09-2016, 11:47 PM   #2 (permalink)
dwrat
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Great info here.
Now I wonder if the results would change at say 70 mph which will change the RPMs a bit?

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Old 09-10-2016, 08:05 AM   #3 (permalink)
Captain Slow
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My guess is, as the RPMs move closer to 2500 RPM the engine is moving too far from the max. torque peak. The H.P. increase may not be enough to compensate for the torque loss.
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Old 09-10-2016, 09:35 AM   #4 (permalink)
CntrlCalDmax
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Quote:
Originally Posted by Captain Slow View Post
.............
Original design strategy by General Motors
My personal assumption is that GM optimized 6th gear (second overdrive) for maximum MPG for a truck that spends most of its life with no load or some load in the bed for the majority of the truck owners. The minority of the owners who really use their truck to pull heavy loads are not given much thought in the finished design concerning MPG. Obviously the “Tow-Haul mode” is a big plus for heavy hauling but the transmission still tops out in 6th gear.............


True, but, not until 65 mph. At least on my current 2012, I don't remember on the 2007. So maybe the General did put some thought into it.
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Old 09-10-2016, 05:20 PM   #5 (permalink)
turnpike
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Good opener..... but I would hope the folks that tow a couple of thousand miles a week could provide an answer, and reasoning.
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'16 Chev 2500HD LTZ Z71, CC, Duramax, Allsion, 4wd, new Sept 2016, has factory fifth wheel prep so added a Curt hitch.

Traded off-'05 Chev, 2500HD, CC, SB, Duramax, Allison, 4WD, bought new April '05, sold at 179,400 miles, LBZ mouthpeice, Blocker Plate & Unplugged EGR, Air Box cut out, lost the cat, 52 US Gal (196 Litre) Titan Fuel Tank, SuperChips tuner reset to stock and retained, AirLift air bags, BrakeSmart Trailer Brake controller, PaceEdwards cover-replaced 2013, DSP Fifth Hitch, JVC KW-NT1 headunit, kept the"AutoEnginuity" scanner.

'02 GMC, 2500HD, CC, SB, D/A, 4wd, Traded in - wrong color
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Old 09-11-2016, 08:05 AM   #6 (permalink)
IGO1320
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I tow in 5th that seems to get me the best mileage. Unless trailer is very small with little drag then 6th
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Old 09-11-2016, 11:49 AM   #7 (permalink)
Ron Nielson
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As a supplement to the mileage discussion, what is the best indicator of better mileage as we drive? Is is Boost? Load? Something else? Perhaps it's not as simple as looking at just one parameter?
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Old 09-11-2016, 12:56 PM   #8 (permalink)
turnpike
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I maybe a bit "old school", but.....
There is a saying that goes something like "if it will not respond to throttle up you are lugging it". That said diesels will lug much better than a gasser. But I find when you have your foot in the fan, you will pay. Maybe drop a gear and let it run easier. When down a couple of gears and the engine is racing, it burns a lot of fuel. It is a balancing act.

It has been pointed out that the Duramax engine's peak torque is at about 1600. So if you are running such that you are at part throttle, and cruising along, you likely are not burning much fuel. On the other hand, if you have your throttle down on the floor.... you will push a lot of fuel through it. Maybe dropping a gear, run the RPM up a bit, ease out of the throttle and save fuel.

There are times you will have to bite the bullet, buy the fuel just to pull against a strong head wind. But over the long haul the way the engine reacts to throttle is a good indicator of your fuel economy.

Lastly, I DO NOT trust the driver information center on short hauls. I do use a pencil to figure the mileage by the tank full (45 gallons.... 600 miles or so). I prefer to figure the mileage on the longer 1500+ mile trips. Day tripping I never worry about the mileage
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'16 Chev 2500HD LTZ Z71, CC, Duramax, Allsion, 4wd, new Sept 2016, has factory fifth wheel prep so added a Curt hitch.

Traded off-'05 Chev, 2500HD, CC, SB, Duramax, Allison, 4WD, bought new April '05, sold at 179,400 miles, LBZ mouthpeice, Blocker Plate & Unplugged EGR, Air Box cut out, lost the cat, 52 US Gal (196 Litre) Titan Fuel Tank, SuperChips tuner reset to stock and retained, AirLift air bags, BrakeSmart Trailer Brake controller, PaceEdwards cover-replaced 2013, DSP Fifth Hitch, JVC KW-NT1 headunit, kept the"AutoEnginuity" scanner.

'02 GMC, 2500HD, CC, SB, D/A, 4wd, Traded in - wrong color
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Old 09-11-2016, 01:31 PM   #9 (permalink)
Ron Nielson
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Quote:
Originally Posted by turnpike View Post
Lastly, I DO NOT trust the driver information center on short hauls. I do use a pencil to figure the mileage by the tank full (45 gallons.... 600 miles or so).
Turnpike

What part of the DIC calculation do you think is erroneous?? Fuel? Miles? Math calculation?

I know that in my truck, the fuel used is very close to what I put in at the pump, IF I am patient enough to fuel the tank to the same level each time - which doesn't happen very often.
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Old 09-11-2016, 08:43 PM   #10 (permalink)
IGO1320
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Quote:
Originally Posted by Ron Nielson View Post
As a supplement to the mileage discussion, what is the best indicator of better mileage as we drive? Is is Boost? Load? Something else? Perhaps it's not as simple as looking at just one parameter?
The "load" % on my CTS tells me what my mileage will be.

__________________
2006 2500HD CCSB 4X4 RED LBZ
Spray-In Bed Liner, Air Bags
Custom Reese 20K slider FWH/Gooseneck hitch
265/75/16 Michelin LTX AT2's on PYO's
Duramax.com tow tune w/turbo brake
PPE Dual Fueler (P0087 cure)
Kitty on a "warm shelf", K+N Drop-in filter
Amsoil in all places, Amsoil BMK-27 Bypass
KYB shocks (stock sucked)
Autometer Boost/Pyro gauges/Edge CTS
EGR blocker and (Kennedy) finger stick
Merchant remanufactured transfer case 748,000
Kennedy lift pump, reserve tank with transfer pump
Suncoast trans and 1700 stall converter @ 748,000
14.558 @ 91.03 Hypertech now?????
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