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YOUR RESEARCHCan anyone explain.......
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Check speed at the end of every mile
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Let's say you are traveling 10 miles, and you check your speed at the end of every mile.
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Following a journey this morning my PCM onboard computer read:
Time: 45 mins
Miles: 9.0
Av mph: 14.5
Surely the mph should be 9 x 60/45 = 12.0 mph
or am I missing something?
Incidentally, what started this train of thought was that at 31 minutes the readings were 6.2 miles and 14.4 mph.
Maths was never my strong point. <img src="graemlins/c.gif" border="0" alt="[ouch]" />
One possible source of the gap: the difference between an average of samples and your overall mph.
Let's say you are traveling 10 miles, and you check your speed at the end of every mile. Let's also say the for the first 5 miles you went 30 MPH, and for the second 5 miles you went 40 MPH. Your average MPH would be:
((5x30) + (5x40))/10 = 35 MPH
Now, do the math for your actual transit time:
5 mi @ 30 MPH takes 10 minutes
5 mi @ 40 MPH takes 7.5 minutes (assume instant transition from 30 to 40)
Your overall MPH would be 10x60/17.5 = 34.29 MPH
Thanks for the suggestions but, sorry, they don't help.
I took particular note whilst stationary in traffic this morning and the average mph is continually recalculated and drops by 0.1 mph every few seconds.
Todays figures were:
Time: 47 mins
Miles: 9.8
Av mph: 14.9
Help anyone?
<img src="graemlins/crying.gif" border="0" alt="[crying]" /> <img src="graemlins/crying.gif" border="0" alt="[crying]" />
[quote]Originally posted by Palting:
<strong>Most plausible axplanation I have heard, Carrerajack.</strong><hr></blockquote>
Unfortunately it's wrong. The sample time is short enough for what you're describing to be insignificant.
For a 17% (2.5mph/12mph) difference in 45 minutes would require a ridiculously large sample time. In reality the sample time is probably less than a second.
[quote]The sample time is short enough for what you're describing to be insignificant.<hr></blockquote>
Actually, the sample rate is irrelevent.
You can see that in the example I posted, it doesn't matter if there are 10, 100, 1 million, or 1 billion samples. The average of the samples will still be 35.
The first method is an incorrect method. You can tell, since you did not need to know time or distance to calculate it. It treated a rate (MPH) as a value.
The second method is correct. For this calculation, all you need to know is the distance travelled and the time it took you to travel it. Two numbers - that's it.
This is illustrated by the classic high school algebra question:
You go on a car trip. For half the distance, you travel at 15 mph.
What speed should you travel for the rest of your trip such that you
average 30 mph?
The answer is that it is impossible for you to average 30 mph - you would have to travel infinitely fast for the second half.
Unfortunately, none of this helps the original poster, unless Porsche has programmed the computer incorrectly!
Carrerajack
But however you look at it, my overall journey time yesterday (Time: 47 mins, Miles: 9.8) should give me an average of 12.61 mph. Yet the PCM computer gave me an Av mph of 14.9.
I am beginning to think Palting is right - its all to do with RHD.
Come on guys, get in your cars, do a journey and tell me if your math agrees with your computer!
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