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Old 05-18-2011, 11:25 AM   #4751 (permalink)
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Join Date: Feb 2010
Location: Ann Arbor, MI
Posts: 53

ElectricZX2 - '98 Ford ZX2 Cool Coupe
90 day: 140.91 mpg (US)
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Some thoughts on mileage

A quick aside/question about fuel economy. I want to calculate mpge but I want to be as fair as I can. I have been averaging about 340 wh/m out of the wall including everything (230 wh/m measuring the dc actually being put into the batteries. This is a mixture of city and highway (70mph) not trying particularly hard to get good economy but driving like a gasoline car (I've seen 180 wh/m into traction pack under optimal conditions).

Anyways using 340 wh/m I calculate 108 mpge but I'm wondering if this number is misleading. The same car got about 25mpg under similar driving conditions when gas powered. Over a 400% increase! But.....

Looking at the big picture in my area we are very roughly 75% coal, 15% natural gas, 5% nuclear, less than 5% renewable. So for a conservative estimate I'm going to assume we are mostly fossil fuel powered with approximately 40% conversion efficiency and 10% transmission loss. So now I calculate my electric car to get:
.4*.9*108 = 39 mpge for my electric car.

I've heard that in the refining/transportation of gasoline you lose about 20%, so really my gasoline version was getting 25*.8 = 20 mpg.

So my electric car is about twice as efficient well-wheel as the gasoline version when using fossil fuels. How you charge obviously has a huge impact on uncle charges his car right next to the hydro dam at lake wenatchee so he's looking much better than me since he's 100% renewable.

Anyways, I just feel like us electric people can be quick to throw out huge mpg numbers that make some of the other hyper-milers look bad. Am I missing something?

On the flip side, the plants that make my electricity are probably on the order of 10 times cleaner in terms of emissions per watt than my gas engine was, and I'm probably emitting slightly less carbon than I used to (since coal emits about twice the carbon as gasoline for equivalent energy...natural gas is much better), but hey, we could use some global warming here in Michigan anyways . So for me its more about cleaner energy, less $ to big oil companies, and better efficiency than huge mpg #s. Plus if I throw up a big wind turbine in my backyard I'll be way ahead!

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