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Old 04-13-2009, 01:12 PM   #1 (permalink)
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Mini test: how ScanGauge calculates average speed

The question came up in this thread about how the ScanGauge calculates average speed.

There are different ways it could be calculated, eg...

Quote:
Is the 32 mph average a true average? eg. x distance / y time = 32 mph or the mean of high and low speeds?
Another way (which I think would be the ideal measurement) is distance/time, but only where speed > 0 - meaning your moving average.

PaleM indicated he's seen his ScanGauge average speed falling while at rest (with the ignition on), so I went out and did a quick 'n' dirty test.

I reset the current trip, drove for 1 exactly minute, coming to a stop at 1:00, then recorded the average speed for 6 more minutes. Surprise - it does indeed fall while stopped, indicating a simple distance/time calculation:

Time - Avg MPH

1:00 - 13
1:15
1:30 - 9
1:45 - 8
2:00 - 7
2:15 - 6
2:30 - 5
2:45 - 5
3:00 - 4
3:15 - 4
3:30 - 4
3:45 - 3
4:00 - 3
4:15 - 3
4:30 - 3
4:45 - 2
5:00 - 2
5:15 - 2
5:30 - 2
5:45 - 2
6:00 - 2
6:15 - 2
6:30 - 2
6:45 - 2
7:00 - 2

My ScanGauge II version is 3.15

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Old 04-13-2009, 01:21 PM   #2 (permalink)
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This means that for my commute, the SG reported average speed is usually about 24 mph. In reality, I'm usually driving in the 30-40 mph range. Stops along the way drop the average.

If you're planning a trip, though, it's actually a more useful metric. Any stops along the way have to be accounted for. With a 60 mph average driving speed, my highway trips are often "rated" at 50 or lower mph. The 50 mph average will tell me the actual trip time, and not some academic-paper theoretical time.
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Old 04-13-2009, 06:39 PM   #3 (permalink)
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Quote:
Originally Posted by PaleMelanesian View Post
The 50 mph average will tell me the actual trip time, and not some academic-paper theoretical time.
Average trip speed is great for planning when to leave; not so useful if the concern is FE.
Average moving speed - down the road - relates more directly to FE.
  • 150 miles @ 75 mph average moving speed; no stops = 2 hous @ 75 mph average trip speed.
  • 150 miles @ 75 mph average moving speed; one 1 hr. stop (lunch) = 3 hours @ 50 mph average trip speed.
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Old 04-13-2009, 06:49 PM   #4 (permalink)
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Good point about moving speed being useful for MPG.
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Old 04-13-2009, 11:51 PM   #5 (permalink)
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OK, may I rephrase? Ideally, I'd like be able to view average speed calculated by different methods.
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Old 04-13-2009, 11:56 PM   #6 (permalink)
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Yes, but can you explain how it is accomplished and what the benefit is compared to the complexity it adds?

The 1 hour break would probably not register. If it is like the guino, the current trip would reset and the tank trip would ignore such a long break (over 7 minutes). Your speed stops counting down after about 5 minutes, so that is probably the SG threshold.
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Old 04-14-2009, 06:43 PM   #7 (permalink)
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Quote:
Originally Posted by dcb View Post
Yes, but can you explain how it is accomplished and what the benefit is compared to the complexity it adds?
No, but I did spend some time mulling it over on last nights drive.

I wonder if gallons/second (well actually either 0.1 gal/sec or 0.01 gal/sec) and/or gallons/second*second wouldn't be a useful (and easily calculated) metrics to see during P&G.
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Old 04-14-2009, 11:16 PM   #8 (permalink)
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Quote:
Originally Posted by dcb View Post
Yes, but can you explain how it is accomplished and what the benefit is compared to the complexity it adds?
Only in pseudocode: if speed > 0, update average

The benefit: it gives a nerd another data point to mull over. Isn't that enough?

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