Differential Fluid for Increased Fuel Econ
10-27-2014, 08:13 PM
#11
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I agree that, optimally, it would be best to run multiple tanks to narrow the error bounds; however, I did take steps to minimize non-mechanical influence (and avoid being shot as the 45s acceleration cases caused some very unhappy people on a 2 lane highway). For pre and post fluid change work drive scenarios, I ran 3 trips at 3 levels of eco driving (0-55 in 45sec, 30 sec, 20 sec) with gear changes as set rpm/speed targets to maintain consistency at relatively equal temperatures (cold start and 50-60F ambient). Since the pre change scenarios were performed on a weekend, I spent ~15 seconds more at lights combined and in the post change scenarios on the weekday, I had to make one additional stop on a 55mph road 2/3ds of the time. Since it takes far more fuel to accelerate a massive vehicle to 55 than to idle for 15 sec, I feel that this would likely benefit the pre-change case. During the weekdays, I kept >200 ft between the front of my car and the other vehicles to minimize drafting benefits. At all three stages of ecodriving, the fuel economy was moderately higher by ~0.8MPG as indicated on the trip computer.
Prior to purchase, this truck was used extensively for towing (w/fifth wheel) on and off road by an oil and gas company in Wyoming, so the old fluid was considerably soiled (the AVG MPG indicator upon purchase was at 10.8 due to the extreme operating conditions). It should be noted that changing the old 75w140 for new 75w140 would likely have yielded a .25-.5 MPG improvement as is, so the lighter weight oil probably only improved mileage by ~0.5 MPG. While non-trivial (2-2.5% of AVG), it is probably not worth sacrificing towing/payload/acceleration capability to achieve this gain for most people. As for a qualitative analysis, the differential is much quieter and there is a noticeable reduction in resistance when coasting down hill or to a light. To quantify this, I did perform a test on a hill near my work. At the top of the hill, I disengage cruise control at a particular sign at 50 MPH, shift to 5th, and record speed at the 40 MPH sign at the bottom of the hill (speed trap). Since this is at the end of my trip to work, the vehicle is fully warmed up and all fluids at temp. Prior to the change, I would roll past the lower sign at 44-45 MPH and after 48-50 MPH, so there is definitely a difference in mechanical resistance (the BMW was 49 with 75w90, 37 with 75w140 as a comparison).
I think the BMW showed such a significant difference in economy compared to the truck with the different weights of fluid due to the AWD system design. Nominally, it tries to shunt the majority of torque to the rear to maintain the typical longitudinal engine, transmission, rear-drive feel when accelerating, but can pull almost all of the torque forward if needed. I think the heavier fluid must have caused it to throw more power forward which killed economy (like driving a truck with 4WD constantly engaged) in addition to the increased drive-train resistance.
To address the concern from MarkD, the mobile 1 fluid contains friction modifiers for LS differential already, so out side of race differentials and performance cars, additional modifier is not typically needed. I have not felt any chattering when turning or driving on loose surfaces, so I believe that the built-in modifiers are sufficient.
Prior to purchase, this truck was used extensively for towing (w/fifth wheel) on and off road by an oil and gas company in Wyoming, so the old fluid was considerably soiled (the AVG MPG indicator upon purchase was at 10.8 due to the extreme operating conditions). It should be noted that changing the old 75w140 for new 75w140 would likely have yielded a .25-.5 MPG improvement as is, so the lighter weight oil probably only improved mileage by ~0.5 MPG. While non-trivial (2-2.5% of AVG), it is probably not worth sacrificing towing/payload/acceleration capability to achieve this gain for most people. As for a qualitative analysis, the differential is much quieter and there is a noticeable reduction in resistance when coasting down hill or to a light. To quantify this, I did perform a test on a hill near my work. At the top of the hill, I disengage cruise control at a particular sign at 50 MPH, shift to 5th, and record speed at the 40 MPH sign at the bottom of the hill (speed trap). Since this is at the end of my trip to work, the vehicle is fully warmed up and all fluids at temp. Prior to the change, I would roll past the lower sign at 44-45 MPH and after 48-50 MPH, so there is definitely a difference in mechanical resistance (the BMW was 49 with 75w90, 37 with 75w140 as a comparison).
I think the BMW showed such a significant difference in economy compared to the truck with the different weights of fluid due to the AWD system design. Nominally, it tries to shunt the majority of torque to the rear to maintain the typical longitudinal engine, transmission, rear-drive feel when accelerating, but can pull almost all of the torque forward if needed. I think the heavier fluid must have caused it to throw more power forward which killed economy (like driving a truck with 4WD constantly engaged) in addition to the increased drive-train resistance.
To address the concern from MarkD, the mobile 1 fluid contains friction modifiers for LS differential already, so out side of race differentials and performance cars, additional modifier is not typically needed. I have not felt any chattering when turning or driving on loose surfaces, so I believe that the built-in modifiers are sufficient.
Last edited by Lucaba; 10-27-2014 at 08:18 PM.
10-28-2014, 08:48 PM
#12
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Most people outside of commercial trucking are in your camp. Might be specious to assume a different lube in a small truck would make much of a difference, but in commercial heavy trucks it is well proven. This is why most factory builds of heavy commercial trucks have full synthetics in trans and diffs. And the OEM's actually extend the warranty by 50% if the user stays with synthetics in those components... from standard warranty of 500,000 miles to 750,000 miles.
It takes several tanks of fuel over a period of time to see if it has any appreciable results. A single run tells nothing, as there are wind and other factors in play. Even driver is a factor. It is well known that when a person does something in the chase for better fuel economy, they tend to also drive a little more conservatively at the same time, thus making it a self fulfilling prophecy. This better driving may not be perceptible by the user, but it is measurable in many instances. In commercial trucking, we take all of these things into consideration to determine cost benefit ratios.
So now you have heard it all!
It takes several tanks of fuel over a period of time to see if it has any appreciable results. A single run tells nothing, as there are wind and other factors in play. Even driver is a factor. It is well known that when a person does something in the chase for better fuel economy, they tend to also drive a little more conservatively at the same time, thus making it a self fulfilling prophecy. This better driving may not be perceptible by the user, but it is measurable in many instances. In commercial trucking, we take all of these things into consideration to determine cost benefit ratios.
So now you have heard it all!
I'm for all synthetics for sure. I even use full synthetic oil in my lawn mowers!
11-01-2014, 10:38 AM
#13
A friend has been running AMSOIL Severe Gear 75W-90 in his 2003 F250 7.3L 4x4 dually for over 5 yrs, every lube is AMSOIL in his truck. He has pulled a trailer coast to coast many times and has not had one problem. It improved his mileage but exactly how much I do know. A buddy of his, who worked at the Ford dealer recommended it, apparently he was using it in his truck for a long time.