75W-140 = lower mpg

[pinehill]

I towed last summer (17,000 lb gross combined) with a fresh fill of 75W-90 synthetic in the rear diff., but I wondered if the 75W-140 wouldn't provide greater high temp. protection while pulling the big hills. So... a while back I changed to the heavier synthetic lube. I also wondered if the heavier lube would affect fuel mileage. Today I found out. Looks like I've lost 1.5-2 mpg unloaded -- same conditions, same driving style, no other changes.

Has anyone else seen a similar effect when changing to the 75W-140?

[mark45678]

I dont think you can blame the heavy oil for the lose of mileage! there maybe a slight disadvantage to useing it like a few tenths to 1/2 mpg in my opinion.I run redline 90-140 in my truck and noticed no change after refill!getting 17+ mpg with JUICE AND 4" system

[csimo]

Makes perfect sense to me that you would drop in mileage with the thicker lube. One of the ways the automakers have used to increase mileage is to use thinner oils.

1.5 - 2MPG seems a little steep for loss, but I suppose it's reasonable.

Once broken in our axles don't produce alot of heat. The lube stays thick and it takes more power to turn it. Just like starting an engine in cold weather - thick oil is hard to move.

[Idle_Chatter]

Well, I had some Amsoil 2000 80W140 lying around, so when I got around to degreasing and fixing my dripping diff (fill plug was seeping and I retorqued all the cover bolts to 40 lb-ft (four on the bottom were not very tight!) I drained out all the "grape juice" and pumped in 3.8 qts of the Amsoil. I was wondering the same thing, as that stuff was like pouring honey and the rear-end felt noticably "tighter" after the change. But...just put in 23 gallons late last night and mileage on that tank---> 20.9!!! A new high record for mileage! Beat out the previous high of 20.5 that had ocurred twice since I bought the truck. I think you need to give it a couple of tanks through before dumping the thick lube! :D