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Old 02-21-2012, 11:08 AM   #1 (permalink)
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Default MID using fuzzy math?

Lately I've been hypermiling to work and back to see what I can do in terms of MPG. Going to work is mostly downhill and flat and I've been averaging 56-58 MPG. On the return trip, I've been getting 42-43 MPG. When I average the two I get a round trip of around 49-50 MPG. However if I don't reset the trip meter, it calculates it at around 45 MPG. It's only a 10 mile round trip but it shouldn't be that off.

Usually per tank, the actual mileage is around 1-1.5 MPG less than the MID.
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Old 02-21-2012, 01:16 PM   #2 (permalink)
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Taking the mean (average) from different mpg figures doesn't work directly, because mpg does not have a linear relationship with fuel consumed. For example, say you take average fuel consumption from two trips of equal length. One trip is 40mpg and one is 60mpg. The average fuel consumption is not 50mpg. It's less.

I think it might work if you convert to l/100km (which is linear), calculate the mean and then convert back into mpg.

Think about it this way - the difference between 0 gallons per mile and 1 gallon per mile is to infinity and beyond. The difference between 0 mpg and 1 mpg is about $3.50 per mile.
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Last edited by Uriel; 02-21-2012 at 01:32 PM.
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Old 02-21-2012, 01:31 PM   #3 (permalink)
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Taking the mean (average) from different mpg figures doesn't work directly, because mpg is not linear. For example, say you take average fuel consumption from two trips of equal length. One trip is 40mpg and one is 60mpg. The average fuel consumption is not 50mpg. It's less.

I think it will work if you convert to l/100km (which is linear), calculate the mean and then convert back into mpg.

I'm going to try that.
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Old 02-21-2012, 02:01 PM   #4 (permalink)
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40 mpg for 50 miles = 1.25 gallons used
60 mpg for 50 miles = 0.83 gallons used

Round trip average for 100 miles = (1.25 + 0.83)/100 = 48mpg

The same for another 10mpg makes it come out even more:
30 mpg for 50 miles = 1.67 gallons used
70 mpg for 50 miles = 0.714 gallons used

Round trip average for 100 miles = (1.67 + 0.714)/100 = 42mpg

This is why the Prius's ability to do low speed in pure electric makes such a difference in city MPG... removing those 15-20mpg parking lot cruises really pushes up your MPG by an unexpectedly large margin.
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Old 02-21-2012, 02:49 PM   #5 (permalink)
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It's kinda messed up the I2 uses so much gasoline at low speed. You can get better MPG at 70 MPH than you can puttin' around a parking lot. WTF?
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Old 02-21-2012, 03:38 PM   #6 (permalink)
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Have you not seen instantaneous mpg in other cars? Previously every car I've driven gets better mpg at a steady 70mpg rather steady 10.
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Old 02-21-2012, 04:15 PM   #7 (permalink)
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It's kinda messed up the I2 uses so much gasoline at low speed. You can get better MPG at 70 MPH than you can puttin' around a parking lot. WTF?
Unfortunately fuel consumption is also a function of the amount of time over which the engine is running, so by going faster (up to a certain extent before air drag comes into play) the time is reduced and so is fuel consumption

Hence why some hypermilers managed to get better mpg by pulse and gliding - when pulsing make the engine work harder instead since no matter how light the throttle it will consume a certain minimal amount of gas; attain enough speed to later glide with the engine off
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Old 02-21-2012, 04:51 PM   #8 (permalink)
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Unfortunately fuel consumption is also a function of the amount of time over which the engine is running, so by going faster (up to a certain extent before air drag comes into play) the time is reduced and so is fuel consumption

Hence why some hypermilers managed to get better mpg by pulse and gliding - when pulsing make the engine work harder instead since no matter how light the throttle it will consume a certain minimal amount of gas; attain enough speed to later glide with the engine off
That's what I've been doing. Getting up to speed quickly and coasting.
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Old 02-21-2012, 04:59 PM   #9 (permalink)
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Quote:
Originally Posted by Uriel View Post
Have you not seen instantaneous mpg in other cars? Previously every car I've driven gets better mpg at a steady 70mpg rather steady 10.
I don't drive around other rides than the ones in my sig.

If you care to buy me a couple random rides here and there, I'm all for it.
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Old 04-07-2012, 10:19 PM   #10 (permalink)
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Default MID versus real MPG

Now that I have put on 20k miles and calculated every fill-up vs gallons on my 2010 Insight I find the MID is 2 MPG higher then actual. I was a bit disappointed at first because I didn't think Honda would design like that. I say only a bit disappointed because now that I am approaching 28k( bought it used with 7k miles) I am starting to hit 52-53mpg on the MID. When I first purchased the car I felt if I can only get 50mpg I would be happy but the first year was only getting 44 to 48mpg actual. Will see what this year brings. My last fill up was 49.7 I'm getting there.
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