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Old 12-04-2009, 12:07 AM   #1 (permalink)
Engineering first
bwilson4web's Avatar
Join Date: Mar 2009
Location: Huntsville, AL
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Managing user expectation - NHW11

When I bought my NHW11 Prius in October 2005, the EPA composite was 48 MPG. Then overnight, September 2008, the EPA moved the composite to 41 MPG. My driving didn't change nor would have changed had it gone the other way. But there is a problem with defining driving skill by a shiftable, arbitrary standard. For example, the European or Japanese mileage standard for the NHW11 are so high as to be unobtainable in practical vehicle driving. The following chart shows the NHW11, 2001-03 Prius, user reported, mileage data from the EPA web site:

What I've done is put vertical lines for various government "mileage" ratings to show how they compare to the distribution of user reported, NHW11 MPG. On the extreme right are the Japanese and European mileages ... quite out of the range of normal drivers.

The problem with an unrealistic MPG standard is the contempt it earns. Many times we've hear or read the old saw that 'You can't get EPA mileage with a Prius.' Then the EPA 'moved the goal posts' and now just about everyone gets EPA mileage.

The old, EPA number is close to the math definition of average + 1 standard deviation. But the new EPA rating, 41 MPG, means all but the lowest 10-15% drivers can meet the "new threshold." The use of average + 1 standard deviation has the beauty of not being set by anything but what drivers achieve with their vehicles.
Originally Posted by note
When I was active at GreenHybrid.com, I once surveyed not only the highest performing owners but also the lowest. I figured both had 'lessons learned.'

One of the lowest performing cars was driven by a medical doctor finishing his residency. He lived in Boston at the bottom of a hill so he never had a chance to 'warm up' the car when he had to go to the hospital. Boston is also frequently cold and hilly and pretty much hybrid-hostile terrain (unless you live on top of a hill.)

I soon realized that +/- 5 MPG is just an accident of the local driving conditions and almost any driver who drives as part of what they must do to live would be stretched to do much different. I approached these folks asking their 'lessons' learned' to better understand and learned I would never challenge anyone about their MPG. I would try to be a resource to share what I have learned from others and verified with my own experiments.
A math definition does not change due to a new government policy. Roughly 1/3d of all driver-vehicle pairs will be above this threshold and 2/3ds below. For me, a 2 to 1 ratio is a reasonable divide between 'efficient' versus ordinary driving.

There is an old engineering maxum that 90% of the budget is spent getting the last 10% of performance. But there are 'low hanging fruit' remaining in the NHW11 and certainly it is a good platform for testing ideas. It nothing else, the Japanese and UK standards are a good goal. <grins>

Bob Wilson

2019 Std. Range Plus Model 3 - 134 MPG3 || 2014 BMW i3-REx - 117 MPGe, 39 MPG
JuiceBox 40 Pro (240 VAC, 40 A), KHONS portable (120-240 VAC, 12-32 A)
Retired engineer, Huntsville, AL (five times AutoPilot saved.)

Last edited by bwilson4web; 12-04-2009 at 05:39 PM..
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