[brucey]

It's almost a given you're sacrificing a few MPG when driving an All Wheel Drive Vehicle. (AWD) I mean, that's 2 more wheels to push and a lot more weight right?

I've always heard it, but never saw any actual tests. I kinda lucked out when I realized I could manually force my car to drive in Front Wheel Drive (FWD) mode by way of a fuse. It disables the rear transfer clutch completely so the car acts like a regular FWD car (albeit, still toting around a rear end, drive shaft, hubs, and axles necessary to be AWD)

So being a good little ecomodder I set out on my test course tonight to see what I could come up with. Test were done by setting cruise control at 50 mph (speedometer and scangauge confirmed) and driving 6 runs, then putting the FWD fuse in, then 6 more runs, then back to normal mode for another 4 runs. Readings were taken with scangauge and after the testing I filled up at 13.9 gallons and the scangauge was within 1% accurate. Car was well warmed up as I had already been driving an hour when I decided to do this.

The course is 1.1 mile straight and level as can be around here, hardly any traffic in the middle of the night.

Temperature was 51 degrees, humidity was at 52% with no reported wind.

AWD:

A: 33.2
B: 33.3
A: 33.2
B: 33.2
A: 33.1
B: 33.2

AVERAGE: 33.2 MPG

FWD:

A: 32.9
B: 34.6
A: 32.9
B: 34.9
A: 32.9
B: 33.4

AVERAGE: 33.6 MPG

AWD 2:

A: 31.1
B: 32.0
A: 31.7
B: 32.1

AVERAGE: 31.725 MPG


This is some really odd data to me. A lot of it is consistent is the strangest part.

The first set of AWD runs I was really happy with how consistent they were. I usually don't get those consistent of numbers on the course.

Then the FWD runs were very consistent (all were identical even) in one direction, but not so consistent in the other. They were all a few degrees higher.

THEN the final AWD run the average dropped again to below the original test average.


Right now however, with that data, I can say that the FWD fuse on an automatic transmission Subaru doesn't do anything to really help gas mileage. Its all within the noise it seems.

Strictly speaking to test this, I should remove all the rear drive train components, but I'm not doing that for a simple test. This really is just to test the fuse itself.

ITS also worth mentioning here, that its my understanding that the transfer clutches give something like 5% to the rear wheels when cruising. I imagine this helps them overcome the drag when lugging themselves. So maybe that's more efficient than just letting them drag behind?