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Hi everyone,
Zak asked me to jump in here since I'm the engineer responsible for EPA range/efficiency testing and generating the MPG data.
The EPA MPG calculation is specified in the Code of Federal Regulations, section 10 part 474. This regulation defines an equivalence between gasoline and electricity of 82.049kWh/gallon. http://frwebgate.access.gpo.gov/cgi-bin/getdoc.cgi?dbname=2000_register&docid=00-14446-filed.pdf
You can then calculate: mpg = kWh/gal / kWh/mi = 82.049 / 0.321 = 256mpg
The 0.321kWh/mi value comes from our range testing that is certified by EPA. However, as Zak correctly noted, the EPA has not had a current procedure for EV testing since 2003. Instead, EPA defers to the procedure specified by CARB, which is the Society of Automotive Engineers recommended practice "SAE J1634 Electric Vehicle Energy Consumption and Range Test Procedure". This procedure uses a chassis dynamometer, and a paraphrased version would say 1) start with a fully charged battery, 2) drive the vehicle over the test cycle until empty and measure the miles driven, and then 3) recharge the battery to full and measure the electricity used.
From our latest testing, the results for city/highway cycles were range of 231/224 mi and electricity use of 0.316/0.327 kWh/mi respectively.
As defined by EPA, you combine the city/highway data using a 55%/45% weighting: 0.321 kWh/mi = 0.55*0.316 + 0.45*0.327
So the MPG number depends on what equivalence you assume for electricity -> gasoline. There are many approaches to this, each with their pros and cons. Some people think that MPG is meaningless for EVs, but there are many others who want to know what the number is. Tesla has decided to quote the mandated EPA 256mpg number. The 135mpg value is an outdated non-EPA estimate, and I'm sure we'll be updating the website soon.
thanks
Andrew SimpsonPosted by AndrewS on May 22, 2008.
It's actually a weird situation. The EPA requires us to use their definition on our labeling, even though we know that the more accurate energy equivalence is somewhat lower. We made the decision to always communicate the higher number with clear labeling that the number is the EPA equivalent. The actual energy equivalent is a bit below the 135 that is currently on the homepage - so we need to update that.
We thought about only communicating what we believe to be the more accurate number, but then you have the problem of the label saying something different and the problem of everyone else using the higher, official numbers. ZENN, for example, quotes about a 250 mpg.
Chalk it up to being hamstrung by the bureaucracy. Somewhere along the way the EPA decided to change the laws of physics to increase the energy density of fuel to benefit CAFE equations for the big companies.
Posted by Darryl on May 31, 2008.
How is that supposed to help big companies with CAFE calculations? All of a sudden everyone looks a lot less efficient.Somewhere along the way the EPA decided to change the laws of physics to increase the energy density of fuel to benefit CAFE equations for the big companies.
Thanks for the info, MDR.
Most references I can find for the energy content in 1 gallon of gasoline convert out to 33 to 38 kWh. 82kWh is quite a jump (over 215%).
I don't really understand this statement by Darryl: How is that supposed to help big companies with CAFE calculations? All of a sudden everyone looks a lot less efficient.
Tesla might want to footnote the MPG figure with an explanation that the number is computed as required by EPA. This would help Tesla's credibility.
Suggested rewording:
"Manufacturers are required to convert from electric consumption to gallons of gasoline equivalent using the Department of Energy equivalence factor specified in the Code of Federal Regulations, Section 10, Part 474, and Tesla has done so. "
ELON MUSK: ... I mean the well-to-wheel efficiency of this car, it uses half the energy and creates less than half the CO2 per mile of a Prius when doing the full well-to-wheel calculation. So if you were to take a gallon of oil, and use that to generate electricity, and take the transmission losses and charging losses and say "well how many miles can the car go?" Or you take that same gallon of oil and you refine it, send the gasoline to a gas station, pulled up a car and drove that car, that's how you do the [...] so it's the same energy source, it's a full apples-to-apples comparison. We get about 135 miles per gallon equivalent, so compare that, that's more than twice what a Prius is.
How 'bout
The EPA requires manufacturers to display electric consumption converted to an equivalent in burned gallons of gasoline using the Department of Energy's equivalence factor specified in the Code of Federal Regulations, Section 10, Part 474.
Extended Interview: Tesla Motors Chairman Elon Musk | Online NewsHour | June 25, 2008 | PBS
"creates less than half the CO2 per mile of a Prius when doing the full well-to-wheel calculation...We get about 135 miles per gallon equivalent, so compare that, that's more than twice what a Prius is."
Sorry, I posted that quote without any comment. Yeah I wanted to point out that I thought 135 MPGe was calculated from the amount of energy contained in a gallon of gas, and not the energy required to refine and transport (etc.) it. Where as the discussion Elon goes through is a Well-to-wheel comparison. My question is does that comparison alone justify the new 256 "MPGe" (i.e. the oil was used to make electricity), or is there even more fudging to it.