Drop in MPG after Rear Diff Fluid Change

[Road_dog]

A few weeks ago while having my oil changed at Jiffy Lube, I was pressured into having my rear differential fluid changed. The tech told me the it was scheduled to be done according to Subaru's maintenance plan, blah, blah... Thinking I was doing what was best for the car, I allowed them to do it. I have noticed about a -3 mpg decrease ever since. I have also notice more resistance when rolling on slight inclines.

 

Tire pressure is good. I tried fueling up at 3 different gas stations, but the problem remains after what is now my 3rd tank. My driving has been fairly consistent throughout this period.

 

The only other changes made was to set the shift light and launch control rpm on my AccessPort. I probably only launched the car twice over 3 tanks of fuel. It's probably almost time for a new set of tires too, but I averaged 25.8 mpg on these tires prior to the diff fluid change. I'm averaging like 22 mpg now.

 

75W90 synthetic was used, but I am having strong doubts about the quality.

 

What do you guys think might be causing the sudden drop in fuel efficiency? My gut is telling me the gear oil in the rear differential.

 

Thank you

[bluesuby]

Since you brought up the J word, are you sure they refilled the diff? Couldn't hurt to check the level by removing the fill plug.

[Max Capacity]

Cold weather/winter gas. My MPG has dropped about the same in the past month.

 

Read covertrussian's thread, he talks about changing the calibration points in the ECU to make it think its seeing warmer temps and his mpg went up.

[Akasa]

First of all, i'd stay far away from Jiffy Lube. I'd recommend doing maintenance yourself. Not only do you save money, but you learn valuable information.

 

That being said, do you know EXACTLY what brand of diff fluid they put in? I know from past experience that our cars (Subarus in general) are very picky about the type of fluids that you feed it. (Gear oil, Motor oil, Coolant)

 

I'm more worried about the increased rolling resistance than the fuel economy.

 

I don't know that I can be of much help, but i'll try to assist if possible.

[Max Capacity]

Yea the rolling resistance has me asking why too.

 

I've always been amazed at how easy my wagon rolls. My Spec B doesn't roll as easy, never has.

[Road_dog]

Thanks for the suggestions. I'm going to check the fluid level and also check my brakes. I read an article where someone had a similar problem because a brakepad slipped out of position. Not sure how likely that is.

 

I usually do most of my own maintenance, but I just didn't have the time that weekend. I figured they were capable of changing a few fluids correctly, but I can't take that for granted. The fact that 3 of 4 valve caps were missing should have been a big red flag. Who knows what else could have been forgotten.

 

Thanks again.

[JLTD]

I've heard too many iffy lube stories to ever go there again. Wrong fluids, no fluids changed, no filters changed, even no fluid. I would have the rear end checked.

 

And IIRC the MD fuels change over some time in late October or early November; if there was going to be a mpg drop it would have occurred weeks ago.