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Old 04-27-2009, 12:59 AM   #13 (permalink)
Ernie Rogers
Ernie Rogers
Join Date: Feb 2008
Location: Pleasant Grove, Utah
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Converting mi/kWh to mpg

Hello, Ryland,

You asked for my take on what your EV car would do as a gasser. Okay, here goes--

Just to repeat myself, there is no right answer for this. My view is that you get a fair estimate of what it will do by multiplying by 17, like this--

3 to 4 miles /kWh x 17 = 51 to 68 mpg

Where did the other 50% go? I threw it away when I converted from heat to electricity-- I assumed a 50% conversion, a little optimistic.

Now, I have a second approach that you might find interesting. When I calculate the mpg of a car using drag, rolling resistance and some drive efficiencies, I can do it for BOTH an EV and a gasser. I looked up one of those calculations and compared the EV numbers to the gasser mpg numbers, and these are the results--

For the very same car, both ways--

3 miles /kWh is equivalent to 45 mpg as a gasser
4 miles /kWh is equivalent to 59 mpg as a gasser

To get these, I used the following drive system efficiencies:

Electric car: 70% of the electricity at the socket reaches the motor shaft
Gasser car: 30% of the gasoline energy is converted to work at the shaft

There is an additional assumption that the car is exactly identical in the two cases in all other respects. For example, I assumed 85% transfer from the motor shaft to the wheels in both cases (a standard assumption for gassers). This assumption can easily be off for an EV with wheel motors or no transmission. (Effects are "backwards" here--lower transfer in the gasser means the equivalent mpgs are lower.)

I hope this actually helps to clarify.
/Ernie Rogers

Originally Posted by Ryland View Post
yes, that light blue vehicle is one two electric cars I own, it's a 1975 Seabring Vanguard, Citi-car, like what TomEV drives, the electric car that I drive every day is a later version, of that car, a 1981 Comuta-car, I don't have any good numbers on the energy usage of it because my batteries are 2nd hand and in rough shape so I've been leaving them on the charger more then I would normally to keep them topped off and to equalize and desulphate the plates, I'm also using the original on board charger from 1981 that does not turn off, it just tapers the charge slowly as the batteries get closer to full, this makes it hard to get exact numbers on energy usage, but if I had to make an educated guess based off Kill-a-watt meter readings at the outlet.
Ernie I would like to see, based off your math, what kind of mileage you think my electric car gets, with my current lead acid batteries I would say I can go one mile on 250-300 watt hours, or about 3-4 miles per KWH as it comes out of the outlet, before the charger.
So what does that come out to in MPG? and at what point am I losing that 50%?
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