Weight matters but it is not everything: We all know that weight is bad for both racing and fuel efficiency, yet some manufacturers have not gotten the "light weight religion" yet. Consider the 5300 pound Fisker Karma for example and it position relative to the trendline shown on the graph. Sure at 5300 lbs, there is no way the Karma will be efficient, but from the looks of the graph, if Telsa, Ford, or BMW had built it, even at 5300 lbs, they would have gotten a lot higher MPGe.
Perhaps the most direct comparison in weight terms is the Ford Focus EV and the CODA sedan which both weigh about 3700 lbs. Ford manages 105 MPGe where as CODA only manages 70% of that at 73 MPGe meaning the CODA really likes to chow down on charge.
Purpose-Built EVs Better, some notable exceptions: Generally, EVs that have been purpose-built, as opposed to adapted, have better efficiency. The from-the-ground-up built as EVs, Volt, LEAF, and Tesla Roadster all have excellent efficiency, where as the conversions of SMART, CODA, and AZURE Connect all have very low efficiency. But perhaps this has more to do with the technical acumen of the companies involved than whether on not the EV shares components with an ICE sister vehicle.
There are some remarkable EVs that were not purpose built as EVs. The Mitsubishi "i" (or i-MiEV) is the second most efficient (112 MPGe) EV available and it is a conversion from a popular "kei" car sold in Japan, although the US version has modifications to pass US crash tests. The third most efficient vehicle, the Ford Focus EV (105 MPGe), is perhaps more remarkable because of its larger size and practical hatchback design. Now if only Ford would set a more reasonable price and start selling them. Perhaps most impressive at 102 MPGe is the BMW ActiveE. As a conversion, the vehicle is very heavy (4000 lbs), yet manages to edge out the much lighter Nissan LEAF which achieves a respectable 99 MPGe. The reason the BMW is most impressive is that they are planning to build a 2700 lbs BMW i3 which, given the efficiency of the ActiveE, may blow the doors off the competition in terms of efficiency.
Looking Ahead: Amory Lovins of the Rocky Mountain Institute in his book called "Reinventing Fire" describes a vision of so-called Revolutionary+ vehicles which combine lightweight design with electric drive trains. One extreme example of that thinking is the Edison2 eVLC test vehicle that recently got a 245 MPGe EPA rating due to its low weight (1140 lbs) and low drag (0.16). While not a production vehicle, it hints of what is possible. A more likely candidate for the showrooms is the BMW i3 (formerly MegaCity) supposedly coming in 2013. At a weight of 2700 pounds, it is about 600 lbs lighter than the Nissan LEAF and will undoubtedly yield impressive MPGe ratings.
PS: As author John Voelcker often points out, MPG is a non-linear scale and it is really better to use some form of "consumption" metric in this type of data analysis. The graph below shows the same data but in terms KWH/100 miles. Note that smaller number are better.
The comments about the MPGe chart still hold true with the Fisker Karma looking even worse than before.
Year | Manufacturer | Model | City/ hwy/ comb. | City/hwy/comb. | Cd | Wt. | ||||
(kwh/100miles) | (mpge) | (lbs) | ||||||||
2011 | Tesla | Roadster | 29 | 32 | 30 | 124 | 112 | 119 | 0.35 | 2723 |
2012 | Mitsubishi | i-Miev | 27 | 34 | 30 | 126 | 99 | 112 | 0.35 | 2579 |
2011 | Nissan | LEAF | 32 | 37 | 34 | 106 | 92 | 99 | 0.28 | 3366 |
2012 | Chevy | Volt | 36 | 37 | 36 | 95 | 90 | 93 | 0.28 | 3781 |
2011 | Smart | ForTwo | 36 | 43 | 39 | 94 | 97 | 87 | 0.38 | 2077 |
2012 | Azure Dyn. | Transit | 55 | 54 | 54 | 62 | 62 | 62 | 3985 | |
2012 | BMW | Active E | 33 | 107 | 96 | 102 | 4000 | |||
2011 | Fisker | Karma | 65 | 52 | 0.31 | 5300 | ||||
2012 | Ford | Focus | 31 | 34 | 32 | 110 | 99 | 105 | 3691 | |
2012 | CODA | Sedan | 44 | 50 | 46 | 77 | 68 | 73 | 3670 | |
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