This post originally appeared on Orphan Road.
Chevy just announced that it can no longer claim 230 MPG for its plug in electric hybrid the Volt, since the EPA has been having trouble coming up with an appropriate formula. It’s easy to come up with a MPG equivalent for regular hybrids, since in the end all of their energy comes from gasoline. And it would be similarly easy for all electric cars – you don’t use MPG, you use something like MPkWh* (miles per kilowatt hour, which is just a crazy enough unit to be appropriate for the US) that tells you how far you can drive per unit of energy you absorb from the electric grid.
The idea behind any formula they use to estimate a MPG in a plug-in hybrid electric is by it’s very nature flawed. You don’t use a drop of gas if you only drive short distances, and you don’t use much electricity if you drive very long distances. The only honest number to use would be both: MPG for the gasoline-only mode, and MPkWh (can we just convert to metric here already!) for the electric only mode.
* suggested pronunciation: “mepkewih” just rolls off the tongue
[update] It looks like the Society of Automotive Engineers proposes using Wh/mi (watt-hour per mile) for the electric part of a PEH (though they will probably cave and just use a MPG). This looks less clumsy than MPkWh and can be pronounced “whammy” (or so I suggest).
Note: this has nothing to do with Seattle’s infrastructure, but it’s been a slow blog week. If pressed I could come up with some justification that involves the Leaf.
Good question, Matt. I wonder if it can be even simpler than that. EnergyStar ratings on appliances tell you how much *money* the appliance will cost you over the course of a year. Obviously they don’t know how many loads of whites you do in a week, so they make a guess based on national averages.
Why can’t we get rid of MPG altogether, and come up with a cost function that takes into account the average nationwide price of gas and the average nationwide price of electricity? How much does it cost to drive this thing, say, 12,000 miles per year? We could do it for every car, not just electrics. You could even do a range.
$/mile (or maybe $/10k mile). Complete with a bar ranging from “Uses Least Energy” to “Uses Most Energy”. Beautiful. They’d have to update it at least once a year as gas prices start rising, but as long as they update all of the ratings at once it wouldn’t matter.
Well, I think you could pretty much do the short form here. The first cost of the vehicle will be so great that whatever you pay for energy is inconsequential. The key question is how far you go on the battery alone- that tells you all about the engineering relative to the vehicle weight, air resistance, and mechanical friction.