Differential Fluid for Increased Fuel Econ
10-10-2014, 04:30 PM
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Differential Fluid for Increased Fuel Econ
I just bought a 2012 F-150 XL 4X4 5.0L w/3.73 LS that has 83,000 miles and would like to swap the rear diff fluid. I will likely never use the truck for towing and primarily use it in snow and for hardware store runs. I know 75w140 is recommended; however, I was wondering if anyone has used a 75w90 or 75w110 and seen any fuel economy improvements (I currently get about 21-22 avg at 50/50 city/hwy - Yes, I drive like an eco-mentalist). I have a 2007 BMW X3 3.0si as well and have tried both in the front/rear diffs and I get 28 MPG avg with 75w90 but only 25 with 75w140 (and it's really sluggish in winter time). I know the truck has a much simpler 4WD system compared to the BMW, so I don't know if the gains would be comparable. Is there any danger in using a lighter oil as long as I'm not towing? Has anyone else tried and seen any gains? Thank you!
10-20-2014, 09:22 PM
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I went ahead and did the swap with some Mobile 1 75W90. I did a 'city' run today with 3 stops in <2 mile radius from my house (no speeds greater than 45). I got 18.5 MPG AVG without using manual shift mode and without too much eco-driving (I could probably hit 19.5-20 if I was being obnoxiously slow). While cruising at 45 on a flat bit of road, I was averaging (per trip computer) 30.8 MPG, so I have high hopes for my HWY mileage to be at least 25-27 at 55 MPH. I'll post an update when I have a chance to drive my normal work route again to compare to the original case. Overall though, the rear end was actually a bit quieter and everything seemed pretty smooth.
10-20-2014, 09:25 PM
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Did you put friction modifier in when you changed the dif fluid? You said you have a LS 3.73. IIRC it requires a friction modifiyer to work properly.
10-21-2014, 04:35 PM
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The friction modifiers in the mobile 1 fluid appear to be sufficient. I don't get any chatter on turns and I took it off-road in a muddy area and it didn't have any hiccups there either. If I do get any chatter, I'll pump a bit of fluid out and add it, but it does not seem necessary at this time.
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10-25-2014, 08:45 AM
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Okay, after testing it out a bit, I am getting 19 city 27 highway and my work trip is averaging 22.5 with moderate eco-driving (super eco-mentalist driving could probably yield 23). The change on the rear differential alone has probably made about a 0.75-1 MPG increase. Hopefully with a good engine tune up, front diff and TC fluid swaps, and a tonneau cover, I can get another MPG and maybe one more when I replace the tires with more road friendly ones vs offroad ones.
10-26-2014, 08:23 AM
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Eliminating friction modifiers is a very risky move. The money you save of fuel will easily be lost when the rear end comes apart. Even changing tires will help fuel economy ,but again the investment in the tires will offset your fuel savings. Checking fuel economy is a long term endeavor, not a trip around town. Tell us in a few months how much you've saved in gas v.s. how much you spent getting there and make a believer out of me.
10-26-2014, 01:30 PM
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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!
Last edited by Cowpie; 10-26-2014 at 01:36 PM.