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Old 10-12-2012, 04:23 AM   #5 (permalink)
wmjinman
Master EcoModder
 
Join Date: Oct 2012
Location: Carson City, Nevada
Posts: 612

Jimmy - '00 GMC Jimmy SLT
90 day: 21.18 mpg (US)

The White Gnat - '99 Suzuki Swift
Team Suzuki
90 day: 51.87 mpg (US)
Thanks: 240
Thanked 109 Times in 87 Posts
OK, one of the reasons I finally signed up for this site today (er - YESTERDAY, now that it's 1am) is because it's the first time I used my new ScanGauge! I have another ScanGauge from a couple years ago in another car (my '99 Swift, or "White Gnat"), but it's on loan to someone else at the moment.

But after reading about doing a bunch of mpg tests on one tank with the ScanGauge (ABA, etc), I was intrigued. So I thought I'd try it on my 2000 GMC Jimmy 4.3 liter 4speed auto. One thing I'm wondering about is whether the ScanGauge is accurate for mpg BEFORE the 2nd fillup "calibration". Because this was immediately after the FIRST fillup (burning through that 3/4 tank so I can "calibrate"!! ;-)

The test I did, however, was very interesting, and the first time I'd ever seen such consistant results. Before, it was the "between fillups" tests, trying to drive a certain way with that whole tank, usually on longer trips, but usually with less than definitive results.

So what I did this time was an attempt to see if there was any clear difference for different speeds. I picked a section of freeway (and adjacent "old highway" for the slower stuff), and did 2-way averages, setting cruise control at "target speed", then hitting "current trip - reset" on the ScanGauge, then driving approx 5 miles on relatively level road, logging the mpg, turning around, resetting "cruise" and "trip", and going the same 5 miles in the opposite direction. This was in Washoe Valley (5200 ft. elevation) and around 64 degr and "light & variable winds". My results are as follows:

speed - n. bound - s. bound - avg. mpg
35 - - - 29.5 - - - 31.8 - - - 30.65
40 - - - 32.7 - - - 31.7 - - - 32.2
45 - - - 30.0 - - - 32.8 - - - 31.4
50 - - - 28.7 - - - 30.5 - - - 29.6
55 - - - 28.0 - - - 28.7** - - -28.35**
60 - - - 27.2** - - - 26.6 - - - 26.9**
65 - - - 25.5 - - - 25.5 - - - 25.5
70 - - - 25.0 - - - 24.7 - - - 24.85
75 - - - 23.0 - - - 24.4 - - - 23.7
80 - - - 21.8 - - - 22.5 - - - 22.15

** During both the 60 mph northbound run and the 65 mph southbound run, I came up behind cars that forced me to slow down for some period, probably interfering with the test's accuracy. However, the results still "looked reasonable" compared to ther rest of them.

As for the surprises; 1) I never dreamed I'd get over 20mpg at 80 mph!!! This is why I wonder if the gauge isn't accurate yet - I've had this vehicle for quite a few years now, and until today, thought I had a pretty good idea what kind of gas mileage it got. ???
2) I never dreamed 40 mph would be the "peak efficiency speed". I always thought it would be around 50, with the mpg dropping below that due to "inefficient engine speeds".
3) I never dreamed it would be such a clear-cut result. I've had some experience trying to do a series of repeated tests to determine a maximum something, and usually get frustrated because the results are usually "inconclusive". But if you plot out these numbers on a graph, the line is VERY definite (and much straighter than I would have expected).
4) That brings us to the surprise that it was so "straight"... I was expecting it to curve over sharply and to drop like a rock at 70 and above, but it didn't - just kept decreasing at about the same rate....
5) And I guess I was a bit surprised that there was a "point" at 40. Again, along with the sharp drop I was expecting at 70 and above, I was almost expecting it to flatten out and be about the same at 45, 50, and maybe even 55 or so. Nope, better and better to 40, then sharply down at 35.

I even repeated the round trip test at 40 mph to see if I could get a confirmation, or if that might have been a "flier". It was even BETTER!!!! (by a little bit). The second one was: northbound=31.9, southbound=32.9, average=32.4

A couple of details. My Jimmy is STOCK and a 4 wheel drive with around 150,000 miles on it. Windows up (closed), roof rack on, mirrors and radio antenna "deployed" normally. (gonna try it with the "mods" soon - AFTER ScanGauge "fillup calibration"). I already have the cardboard cut out for grille block & rear wheel skirts. Have one WalMart pizza pan for one front wheel, just need to pick up the second. AND I have couple pieces of plexiglass I need to bend in the oven for "aero" headlignt covers.

But who knew the ol' Jimmy would get 32 mpg at 40 mph?!?!?!? This is exciting! Of course, driving across Nevada at 40 mph might not be too exciting!!! HAH!!

Last edited by wmjinman; 10-13-2012 at 03:20 AM.. Reason: correcting spelling
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