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Old 10-12-2012, 09:56 AM   #11 (permalink)
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Originally Posted by wmjinman View Post
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....
That sometimes happens. Even though the aerodynamic load is going up dramatically as speed increases, depending on gearing and the engine in question, the engine may be be getting closer to its peak high-load efficiency zone - see Brake Specific Fuel Consumption (BSFC) Maps - EcoModder

Approaching peak BSFC as speed climbs would tend to "flatten out" the line on the chart. (Moreso than you would see with an electric vehicle, by comparison.)

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Old 10-13-2012, 03:53 AM   #12 (permalink)
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Originally Posted by euromodder View Post
Most likely not.
Though the relative results will still remain after calibration.

The results from 35-45 seem to be in the same ballpark, though it could well be your car's most FE speed is 40.

At reasonable speeds though.
Depending on gearing, it may well be 20 mph ... at idle rpm in a high gear.

Road-testing for FE is tricky, as things change along the way - often beyond your control.
What we perceive as flat isn't always so flat either
To do this really right, one'd have to measure the exact same stretch of road every time (from a milepost or a sign to another such landmark).

Site-starter MetroMPG has a thread on how to do road testing the right way - or as right as the road will allow you to test - (http://ecomodder.com/forum/showthrea...ery-11445.html )

Personally, I don't do dedicated tests.
I do try out things on my regular commute.
If it works, it'll show on the gauges and ultimately show up in the fuel stats.
Thanks euromodder,

I followed the link to MetroMPG's thread and read the entire thing on the proper way to do testing. Very enlightening. Obviously, my "single pass each way" wasn't very scientific, but I think I still gained some valuable information from it.

Regarding your caution about using the same section of road each time to a certain sign or milepost, I did that. Also, I did it both directions, so any deviation from "true level" should be compensated for, as well as any steady wind from any direction. And while the course was not level, and did, indeed have minor hills, I believe going both directions should have given me a pretty good average. And it was 5.3 miles (over 8 km) from one end to the other, so I think it was far enough to take out random little events.

Finally, my main purpose was to see if I could discern any difference in mpg with speed (boy, did I!), and furthermore, what seems to be the "best speed". So while I won't take the absolute mpg numbers as gospel, I think I now have a pretty good idea of how my car's gas mileage changes with speed.

Once I'm down to 1/4 tank, I'll do the "fillup calibration" on the ScanGauge, and go repeat parts of the test for comparison, probably just 40, 50, and 60 mph if there seems to be a good correlation to the first test. My "real" goal is to start testing my mods, rather than come up with exact mpg figures for different speeds in "stock" configuration.


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