3 different ways to calculate MPG, which one do you prefer?

W

Wayne Wilson

Member
I recently went 858 miles between fill ups, so a lot of EV driving. The fill up was 4.6 gal and the total electricity used was 197 KWH.

149 MPG or 84 MPG or 62 MPG.

The Clarity's trip computer showed 189 MPG.
Using the government energy equivalence of 33.7 KWH to a gallon, I get 84 MPG.
Those 197 KWH's at my average electricity cost would have bought 9.2 Gallons at today's fill up price, for a price equivalence of 62 MPG.

Of course the first is just a fantasy, but between the latter two, which one to use when people ask about my fuel economy?
 
I like what you said first - just as you said it. Let others do whatever heavy math lifting they'd like if they so desire.

Really, it's about how the car is driven rather than one fuel efficiency number that will tell all.

Which one makes most sense for your situation?

Your mileage may vary…
 
As far as I'm concerned you got 186.5 MPG. You drove 858 miles and used 4.6 gallons of fuel, that's what MPG is - miles per gallon.
If you want to throw electricity into the calculation it's something else - maybe miles per dollar, or miles per BTU. Personally I think miles per dollar is the more interesting of those, but I also factor in the convenience of not having to go to a gas station.
 
I like the idea of cost ie miles per cents, see my other thread on mpg and mpc. Converting cost back to mpg is reasonable as it makes things bit more uniform for all of us to compare, as we have different rates for both electricity and gas.

That leaves driving style and terrain and traffic pattern that are unique for everyone.

I find plugging in the car every day as a mundane, but extra chore as opposed to a weekly five minute fill up at the gas station which I don’t mind. I knew this from the get go, but it adds one more thing to daily routine.
Bit more annoying for me as I charge at work and I need to move the car out of the charge stall after four hours, so one more thing to remember, walk to the garage, find another parking spot which can be bit tricky sometimes. Oh well
 
First, it is important to note on gas car, MPG is a measure of efficiency. How much energy is my car using, and how does it compare to another?

The 189 mpg is marketing BS figure. I would say I used 4.6 gallons of fuel and 197 kWh to drive 858 miles. They could be converted to MPGe to show overall energy use. This blended MPGe can be compared, but is highly sensitive to amount of electric used.

I think the only accurate answer to the question: "how efficient is my car?" is 82 MPGe. That isn't useful to you for cost purposes, so knowing Gallons and kWh used independently is the bare minimum info I want to see.

It is easy for me to track gallons when I fill at pump and with power meter on car EVSE I can track kWh.

MPGe is efficiency of car, absolutely nothing to do with costs.

kWh and gallons are used to calculate costs, nothing to do with efficiency (but with miles can calculate efficiency)
 
The reason MPGe is the way to compare is this, pretend you have a 100 mile drive and you use a Clarity and use 1 charge (14 kWh) and 1 gallon of gas to drive it vs on a Prius Prime you use 1 charge (6 kWh) and 1.5 gallons of gas, the overall MPGe in those two examples is 70 mpge for Clarity and 60 mpge for Prius Prime, meaning the Clarity is using less energy in this example case.

If you are concerned which is cheaper, maybe gas is $3.00 and electric $0.1, then Clarity is $4.40 and Prime $5.10 in this example case.
 
I think the 3rd where you use cost can't really be used for MPG comparison with the other 2 methods. Cost should not be involved with the other 2 calculations as they do not incorporate cost. Imagine electricity being the same cost but gas being double the cost. Your MPG changes for the 3rd calculation, but not for the other 2. Same could be said for just ICE calculation, cost does not bear any impact.
 
Okay, far warning, this is a pet peeve of mine...

The term "MPG" has a very specific meaning: "Miles Per Gallon". As in, the fuel efficiency of a gas-powered vehicle expressed in the number of gallons of gas it takes to propel the car down the road a certain number of miles.

It does not mean "Miles per one gallon of gas plus an indeterminate and unreported number of kWh of electricity".

Wayne Wilson said: "I recently went 858 miles between fill ups... The fill up was 4.6 gal and the total electricity used was 197 KWH."

So then, you got 858 miles from 4.6 gallons of gas plus 197 kWh of electricity. In a PHEV, energy use from gasoline and energy use from electricity should always be tracked and reported separately, just like that. Any other method of reporting is just confusing the issue, even if the car's instrument panel reports this as "MPG". That's not real MPG, it's fake MPG!

Down off my soap box now. ;)

Viking79 said:
The 189 mpg is marketing BS figure.
Click to expand...

I was trying to be more polite than that... but yeah. So is this old ad for the Chevy Volt, for exactly the same reason:

chevy-volt-gm-230-mpg-photo01.jpg
 
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I agree with tracking electrical use and gas use separately. I've drive about 975 miles in EV mode and 25 miles on HV mode. I am averaging 34 KwH per 100 miles and 45 MPG. The 34 KwH is based on the KwH that is being drawn from the house. I am drawing electricity from a Siemens 32 AMP EVSE at hom3, and from what I can tell, it is 92% efficient.
 
I convert the cost of electricity used to the amount of fuel that would buy. Compare that to the distance traveled and you get a cost based MPG that people who have gas powered vehicles can relate to.

Ie. 38.5 miles for $2.14 in hydro
$2.14 buys 0.43 US gallons
38.5 miles divided by .43 gallons equals 89.5 mpg (cost based)
So in terms of what is costs I am averaging 90 MPG for an Accord sized vehicle. Sweet.
 
Pushmi-Pullyu said:
In a PHEV, energy use from gasoline and energy use from electricity should always be tracked and reported separately, just like that. Any other method of reporting is just confusing the issue, even if the car's instrument panel reports this as "MPG". That's not real MPG, it's fake MPG!
Click to expand...

I think there is a valid argument to be made for combining them and reporting MPGe as a blended measurement. MPGe uses the potential energy contained in a gallon of gasoline to come up with an equivalent efficiency rating for electric vehicles. So, in the example above, the 84MPGe figure allows you to compare "apples to apples" in terms of your drive's efficiency compared with other vehicles or other drives you make.

Another way to look at your efficiency would be to examine carbon impact. For example, assume that the above drive were taken in my state, Indiana. According to the Energy Information Administration, Indiana's carbon dioxide emission rate was 1.8 pounds per kWh in 2016. Multiply that by 197 kWh and you get 363.7 pounds of CO2 from the electric side. Assuming your gasoline was 10% ethanol, it produces 17.6 pounds of CO2 per gallon when burned, so the 4.6 gallons of gas created 81 pounds of emissions. So our total CO2 impact for the drive is 444.7 pounds of CO2 over 858 miles, or 0.52 pounds per mile.

Note that the above numbers include the "long tailpipe" of carbon emissions from electricity generation, but exclude those from gasoline extraction and refining, so the numbers for carbon emissions per gallon really ought to be higher. But I couldn't find better data quickly enough for this post.

Anyway, the point is that which number you use to measure your efficiency really depends on what it is you are trying to measure. If you are trying to measure literal miles per gallon of gasoline, then Honda's numbers are "correct". If you are trying to compare efficiency across drives, then MPGe is the way to go. If you're concerned about cost, then you want to measure cents per mile. If you want to measure environmental impact, then carbon emissions per mile are the "correct" measurement.

Sources:
https://www.eia.gov/tools/faqs/faq.php?id=307&t=11
https://www.eia.gov/tools/faqs/faq.php?id=74&t=11
 
Klong said:
I convert the cost of electricity used to the amount of fuel that would buy. Compare that to the distance traveled and you get a cost based MPG that people who have gas powered vehicles can relate to.

Ie. 38.5 miles for $2.14 in hydro
$2.14 buys 0.43 US gallons
38.5 miles divided by .43 gallons equals 89.5 mpg (cost based)
So in terms of what is costs I am averaging 90 MPG for an Accord sized vehicle. Sweet.
Click to expand...

But what if it cost $2.14 buys you only .10 US gallons. Same 38.5 miles
38.5 miles divided by .10 gallons equals 385 mpg (cost based).

How can you have 89.5mpg become 385mpg and try to compare it to an ICE car or even the same car when prices fluctuate. If you want, as a previous comment stated, Miles per cost, then that makes sense, but not MPG.
 
I find MPGe so misleading it should be abolished. We should keep seperate measurements for each type. Mpg for gas and miles per kwh for electric . Trip computers should have combined miles, electric miles and gas miles . Easy as pie

First step is adopting the metric system...
 
I think that HV mode complicates things and sometimes requires a blended number. This is because you can use both electricity and gas at the same time and on the same trip. For that period of time a blended number is really the only thing that makes sense, even if it seems to be impossible to formulate rationally. If you drive very little in HV than the net amount of electricity consumed during your drive while using HV is very small compared to the amount used in just EV mode and your mileage approaches pure EV. Conversely, if you drive primarily in HV, whether on long trips or just for the fun of it :) the net amount of electricity used compared to gas is very small and has little impact on the overall efficiency and the more your mileage approaches a traditional, albeit hybrid, car. Somewhere in the middle of these usage patterns is really mixed up.
 
Wayne Wilson said:
If you drive very little in HV than the net amount of electricity consumed during your drive while using HV is very small compared to the amount used in just EV mode and your mileage approaches pure EV. Conversely, if you drive primarily in HV, whether on long trips or just for the fun of it :) the net amount of electricity used compared to gas is very small and has little impact on the overall efficiency and the more your mileage approaches a traditional, albeit hybrid, car.
Click to expand...

Exactly, thank you. As you say, the kW used could be a small part of the overall mix of energy use, or it could be a lot, for any given trip, or for any individual driver's driving habits.

That's why just reporting the gas usage in a PHEV as "MPG", reporting gasoline use without also reporting the kW of electricity used, makes the number meaningless. As I said, fake MPG rather than real MPG.
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