We own two Prius, an 03 and 10, and the 03 Prius had the EPA composite mileage change in September 2008 from 48 to 41 MPG. But it was getting the same 52 MPG before and after the change. Yet scoring based upon an EPA ranking gives this vehicle a much higher higher and unearned ranking by 'moving the goal post.'
Another scoring approach is to multiply the MPG times the internal volume to give a utility ranking. This is the cubic feet.miles per gallon, a measure of how much payload was moved a given distance per mile. This chart shows the two scoring mechanisms:
The utility scale was adjusted to overlay the EPA based ranking of the NHW20 Prius.
There were five models in the rally:
- full size sedan - Camry hybrid, lower ranking
- full size hatchback - NHW20 Prius, stayed same
- full size hatchback - VW30 Prius, higher ranking
- compact - Scion xA, big drop
- compact - Jetta gas, big drop
The EPA based ranking gives no indication of the relative size of each vehicle so the Scion xA and Jetta compact cars had a high ranking but small payloads. In contrast, a utility ranking, a type of 'price-performance' ranking, shows how much service each vehicle provided for a given amount of gas without impacting relative driver skill. It normalizes the different vehicles.
A utility ranking also opens up competition for a large volume vehicle with a small engine ... say a delivery or semi-trailer truck. However, this is handled by giving passenger volume a higher weight and clamping the cargo volume to some ratio of passenger volume. This would keep the Camry hybrid from decreasing so much. But in theory, a well design bus (some of the earliest hybrids were busses) could compete. Although I haven't done the math, even motorcycles and mopeds could compete. This type of universal ranking begins to approach the 'seat-mile' used to evaluate flying versus driving.
There is a practical aspect of a utility versus EPA based scale. Imagine you are in New Orleans and have a fixed amount of fuel, a thousand people with suitcases, and need to move them away from an impending disaster. The utility scale allows us to select the vehicles and drivers to optimize the task. An EPA scale would leave people behind.
I know how the historical, EPA scale came into being. Just it may be time to find a scale that is independent of changing, parochial, government regulations and based upon the intrinsic characteristics of the vehicle.
Thanks,
Bob Wilson
ps. Added the spreadsheet if anyone wants to look at alternate scoring ... say mass, paint color, rain drops . . . Ok for those entertained by numbers.