wow, now that's a drop in gas mileage.....I ran the 75W-90 Mobil 1 before I installed the 75W-140 and haven't noticed any difference. I doubt it's the rear-end gear oil causing your mpg drop. Did you fill up at a different gas station or has it been extremely cold where you live?
I would give it another few tank fulls before the red flag starts to wave. You very well could have a problem but I'm 99.9% sure its got nothing to do with the heavier gear oil your running. Please keep us updated as to how your truck does on a couple more tanks of fuel. Also keep good records if you have to make an appt. with the dealership. At least then you'll have some sort of documentation to show them the drastic drop in mpg your seeing.
One other thing I would like to add is if your gas mileage stays this low, the dealership can update your ECU to the latest revision. This may or may not help, but I do know updated revisions are always coming in for our ECU's. Just a thought........So again, please keep us updated and good luck!!
naaaaaaaa, forget what I just said, with more thought on this I'm now 100% sure it has nothing to do with your gear oil. To drop 4 mpg you would have to be towing 4 to 5K lbs. and I can assure you the gear oil is not putting that much drag on your driveline.
How about some other possibilities:
1. Do you have a locking gas cap?? Locals could be siphoning off a little trying to make sure you don't notice it. That is if you park outside.
2. A drop in temps. usually causes a slight drop in mpg along with reformulated fuels and different winter time additive packages in fuels.
3. Idling a little too much??? Maybe???
4. The list goes on and on....
Anything you can think of, besides the gear oil, that would have caused this drop??
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