MPGe -vs- MPD : The Real Figure of Merit?

[Al Sherman]

157 miles/1.2 gallons = 130.8 MPG

So, a gas car would have to get 130.8 MPG (or 1.8 liters/100 km) to cost the same as the Model S in this scenario.

This is how I look at it. It's really the same as cents per mile. At 3.3 cents per mile assuming 4 bucks per gallon of gas (it's actually higher here now) I get 122 mpg.

 

[GSP]

Here is a much better figure of merit: the "eGallon"

The eGallon: How Much Cheaper Is It to Drive on Electricity? | Department of Energy

http://energy.gov/sites/prod/files/2013/06/f1/eGallon-methodology-final.pdf

GSP

 

[scaesare]

... I like MPG or MPGe. The kWh or gallon is a unit I can visualize. I know how much it costs. What I want to know is how far I can go on that easy to identify with unit of energy.

But that's the problem. MPGe doesn't tell you that. It tells you what the the car can theoretically travel if using the same amount of energy as contained in a gallon of gasoline. Yet the car neither uses gas, nor does that equivalent amount of electricity it DOES use cost similar to acquire.

This is the formula to determine it:

It's really an efficiency comparison thing (that in my opinion confuses people) rather than a "how far can I drive per unit" or "how much will it cost to drive" metric.

 

[Al Sherman]

But that's the problem. MPGe doesn't tell you that. It tells you what the the car can theoretically travel if using the same amount of energy as contained in a gallon of gasoline. Yet the car neither uses gas, nor does that equivalent amount of electricity it DOES use cost similar to acquire.

This is the formula to determine it:

Yes. However, I don't think he's technically using MPGe. He's using cost per mile for the EV and figuring how many miles he could drive for the price of a gallon of gas. I like this method. I think it's fair other than vampire loss and inefficiencies (use whatever deduction seems appropriate if you like) and don't think it's misleading while people can easily understand it.

 

[wycolo]

> Then there is the vampire losses. [dhrivnak]

So if you are trip-centric you should include vampire days ahead (or behind) each trip when you calculate total cost per mile.

If you are month-centric you are getting useful figures which could then be further improved (or at least simplified) by having a dedicated wh meter. I have one somewhere; need to hook it up!
--

 

[TI Sailor]

There's a reason chain grocery stores compare total costs rather than incremental savings on cereal, laundry soap, etc - Big numbers have a greater impact. In that vein, I will use the following to describe our expected travel (fuel) costs this summer and next:

This summer we will take the Lexus on our 2000 mile round-trip to visit family and friends. Since most miles will be on interstate highways, I expect to average 25mpg, leading to 80 gallons * $4.00/gal = $320 total fuel costs. I won't even throw in the cost of the oil change before the trip, or the possible higher price of gasoline.

Next summer, after the superchargers have been rolled out on the east coast corridor, we will take the Model S and save almost, if not all, of that $320.

Bottom line: On just 3 summer trips in the Model S, we will save $1000. Now, that's a sale's pitch. Easily understood with high impact.

 

[derekt75]

Who cares? I know what a gallon of gas is and how much it costs. What I need to know is how far that will take me. In either case, I know the higher number will take me further for the same cost. In real life, I wouldn't care about which example is a better savings expressed as a percentage.

I care.

Folks [like us] get all excited about making a fuel efficient car even more efficient, but the reality is that fuel efficient cars aren't the problem and making them more efficient isn't a major factor in the solution.
If someone came out with a 500 MPGe car, that would sound really impressive until you realize that that car really helps hardly at all compared to our 90 MPGe cars. It wouldn't help the environment much, nor would it help your wallet much.
Getting those 15 mpg SUVs to be 20 mpg vehicles is actually much more important to the environment.

but environment aside, I think it's usually more natural to have the constant in the denominator, so that you multiply to get the answer to your question. If the question is "how far can I get on a gallon?", then I agree, mpg is sensible. but that's not usually the question. The question is usually "how many gallons will I burn this {trip,week,year}"? In the unlikely event that you know how many gallons are in your tank and how far it is you need to drive, either formulation is equally suitable.

In real life, I might care how many dollars I save by switching cars. I drive 1000 miles per month, so a figure of "dollars per 1000 miles" or "gallons per 1000 miles" would be pretty simple.

 

[mknox]

but environment aside, I think it's usually more natural to have the constant in the denominator, so that you multiply to get the answer to your question. If the question is "how far can I get on a gallon?", then I agree, mpg is sensible. but that's not usually the question. The question is usually "how many gallons will I burn this {trip,week,year}"? In the unlikely event that you know how many gallons are in your tank and how far it is you need to drive, either formulation is equally suitable.

For me, the "constant" is the unit of fuel... I can visualize it, I know how much it costs etc., so that's what makes sense to me. An arbitrary distance as the denominator just doesn't do it for me. The fact is, at the end of the day I can do math and convert from one method to the other, so maybe I'm putting too fine a point on all of this. :smile:

 

[derekt75]

For me, the "constant" is the unit of fuel... I can visualize it, I know how much it costs etc., so that's what makes sense to me. An arbitrary distance as the denominator just doesn't do it for me. The fact is, at the end of the day I can do math and convert from one method to the other, so maybe I'm putting too fine a point on all of this. :smile:

I drive about 1000 miles per month and would like to know how much fuel I need to do that.
I don't say to myself that I'm going to burn 40 gallons per month, and then ask how far that'll get me in this vehicle.

That's why I said that distance is the "constant", not fuel.
If division is as easy for you as multiplication, then it doesn't matter.

Anyway, I just wanted to point out that I personally prefer energy per distance. I like that my car reports 400Wh per mile, instead of 2.5 miles per kWh.
I'm not alone:
Why We Should Measure by Gallons per Mile, Not Miles per Gallon - Popular Mechanics
The Illusion of Miles Per Gallon - NYTimes.com
http://www.sciencedaily.com/releases/2008/06/080619142118.htm
etc., just google "gallons per mile" (with the quotes)

but you're right; either system works well enough, and whether we use mpg or gpm is not as important as whether or not they use miles per dollar instead of miles per gallon (although I'd still prefer cents per mile. :tongue: )

 

[Rodolfo Paiz]

I think it's very important to make the unit of comparison either a dollar, or a mile, because both are fixed constants that are highly important to people and whose cost does not vary. Dollars per mile or miles per dollar are both good, and arguing over whether we should compare per 1 mile, per 100, or per 1000 is just nitpicking. MPG or MPGe both suck because they are only partial comparisons and thereby favor the ICE by not showing the full difference.

 

[Robert.Boston]

I think that cents per mile is the most meaningful for me, personally. It helps me understand how much more it costs me to drive dinner in Portland rather than Bath (Maine), leaving aside the value of my time.

 

[brianman]

I think that cents per mile is the most meaningful for me, personally. It helps me understand how much more it costs me to drive dinner in Portland rather than Bath (Maine), leaving aside the value of my time.

That's currency tied though, so it'll vary from country to country.

How about the gold standard? Ounces per mile.

 

[Rodolfo Paiz]

That's currency tied though, so it'll vary from country to country.

How about the gold standard? Ounces per mile.

Still effectively currency-tied, since most of us think about gold as a store of value and therefore we reflect on its currency-based price in our preferred currency. Besides, I get the feeling that 34,550 miles per ounce of gold is not going to be a stunningly useful metric... ;-)

 

[brianman]

Besides, I get the feeling that 34,550 miles per ounce of gold is not going to be a stunningly useful metric... ;-)

I think that's an awesome sales pitch.

 

[Blaze]

Okay, here are some real numbers. I picked up my S on March 12, 2014. I have a dedicated second meter for the EV. I have received on first electric bill for useage fron 3/12 through 3/19 (1 week). The bill was $21.19 which included $6.00 Service Connection charge. The actual energy was $15.19. I have been spending $150.00 per week in gas. 90% savings! Nice.