Rethinking gear oil fill level

[Reserector [Veteran Member]]

After seeing this, I plan to stop overfilling my rear diff just because it is angled up several degrees. I think the extra oil is just costing me fuel efficiency.

This is an eye opener.

https://youtu.be/9-yG3D3JBRs

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[83chevyk5 [Veteran Member]]

Makes me think about the shape of my diff cover and what all its effecting....

[Green Gorilla [Veteran Member]]

I saw that video and the other two that he did. Some people on a Ford forum were talking about it
https://www.youtube.com/watch?v=n4DQ-jzKQRw
https://www.youtube.com/watch?v=09JsDorR6tw

[Reserector [Veteran Member]]

I ordered three quarts of Red Line GL-5 Gear Oil 57904 from Summit. Not a cheap way to go, but Redline MT90 was so amazing in my Nissan transmission, that I trust the brand, and feel it will be a good investment.



My axle calls for 2.85 quarts. Factoring in for spillage, three quarts should be just right.

I will check the temperature after a trip home from work and compare before and after. I will also compare fuel economy since I think this is a major contributor to the mileage decrease I experienced after the V8 swap. Stay tuned...

[Reserector [Veteran Member]]

I did the fluid change this Saturday. I forgot to check temperatures, but I am monitoring mpg.
This oil flows much easier than the 80w_90 dino oil that was in there. That alone could improve mileage. I suspect it has superior film strength.
I drained four quarts, and refilled with three, which is technically overfilled by 0.15 qts.

I should know something later in the week. My last mileage check was 14.4

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[Reserector [Veteran Member]]

You can't argue with facts, and here they are. I used to get right at 14 miles per gallon, but as my tires wore that number dropped all around 13.8 consistently.

Since I changed the type and amount of oil in my rear axle, I have already gotten to tank bowls right at 15 miles per gallon. I haven't gotten that since I was driving this drivetrain as a Chevy. I've always wondered where that mileage went to and now I know.

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