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Thanks Phil,
I definitely agree that there is no way to draw any solid conclusions from the drive, at least not the kind of conclusions one could get from using a wind tunnel. But I believe I've learned a few things, many simply because I had instant feedback on the Scangauge (invaluable) and could 'experiment' while driving.
The obvious learnings (I knew these but confirmed them on this trip):
- An airbrake under the car is bad for FE
- Increased speed is bad for FE
- Diesel #1 is bad is bad for FE
- Driving downhill is good for FE
Then some less obvious ones:
- AC is bad for FE, but not nearly as bad as I thought it would be. The difference in instant FE on the scangauge was only slightly lower when AC was on vs off.
- Being behind a big rig, even when it's back far enough to see the rig's rear view mirrors, helps FE. Obviously within 2 car lengths would have been better, but I didn't feel comfortable that close. Of course, strong winds gusting perpendicular to the direction of travel negate any advantages.
- Warm / hot ambient air temps are bad for FE. More than I expected. This is a tougher one to nail down though... whenever the ambient air was warm, I was also running the AC. But it seemed that warmer intake air (I think the SG gets the measurement from a sensor that is after the turbo and intercooler on the Jetta) translated to lower FE fairly consistently. Turning off AC didn't change FE noticeably (the change was within the noise of the SG instant FE) but the intake temp started to drop and then the FE started to increase. Not by much, but a little. Turn on the AC, no immediate change in FE, but then intake temp would go up and FE would go down a little. Of course, outside conditions where changing constantly (air temp, road surface, other vehicles, slope, etc etc), so it's not something I'd hang my hat on. But the Diesel seems happiest when the engine is hot and the intake air is cold.
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