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Discussion Starter · #1 ·
I just wanted to confirm I'm understanding the economy summary screens correctly on my car. I just figured out that depending if you select "mpg" or "combined" in the Energy Info menu then it changes the summary you get when you shut off the vehicle. Below are the two summary screens after a short trip (sorry for the glare). My questions are:

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1. The first summary shows actual "MPG" which I understand is to be taken literally as "per gallon of gasoline". So if I drove purely on electric, I burned 0 gallons of gas and thus my MPG is "250+" (essentially infinite).

2. The lifetime MPG is the total gasoline burned since the car was new, divided into the odometer reading. So theoretically if I never burned gas again, the lifetime MPG would just keep going up as the miles increased? (hypothetical question--I know the ICE will run on its own schedule to burn gas in the tank).

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3. The second screen is the combined efficiency. On this particular trip I got 102 MPGe, which is a derived efficiency based on a fixed equivalent mpg (97 mpg?)?

4. The lifetime MPGe is an average since new. Every time I drive I get a summary showing 100-130 MPGe, so if the average was 69.6 then the previous owner really didn't drive for efficiency? Or is this on par with east coast/mid atlantic climates with both cold winters and hot summers?

Thanks,
-Michael
 

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Regarding fuel mileage (average distance per unit of fuel), which is the flip side of fuel economy (average fuel units used per unit of distance), for the Volt, MPG, MPGcs, and MPGe represent different values. Note the Volt has two driving modes, Electric Mode (Charge Depleting, or CD mode) and Extended Range Mode (Charge Sustaining, or CS Mode).

MPG (all caps) = (total electric miles + total gas miles) / total gas used

MPGcs = gas mileage in Charge Sustaining mode = total gas miles/total gas used

The use of the lower case "mpg" to talk about a Volt’s gas mileage is ambiguous unless it is clear from the context if MPG or MPGcs is meant.

MPGe = (total electric miles + total gas miles) / (total Ge gallons of gas consumption) + (total Ge gallons of electric consumption), where 1 Ge = the energy content of 1 gallon of gas = the energy content of 33.7 kWh of "from the wall plug" electricity.

The Gen 2 Volt’s energy usage screen splits the trip combined MPGe into the two components: electric MPGe and Gas MPG (and because 1 gallon of gas = 1 Ge of gas, the displayed Gas MPG = Gas MPGe).

Note the Combined MPGe on the left of the display. This is the calculation:
total trip distance / (Ge of electricity used) + (Ge of gas used)

In your screen shot, you’ve used ~0.0357 Ge of electricity and 0 Ge of gas, so ~3.6/0.0357 = ~102 MPGe combined (allowing for rounding off of displayed distance and kWh numbers). Lifetime numbers are similarly calculated... odometer miles / total Ge from the wall + total Ge used. Lifetime MPGe can drop quickly with a small amount of gas use because, as a propulsion fuel, the energy content of electricity is so much greater than the energy content of gas.

Lifetime MPGe after driving 53 electric miles on a full charge (0.5 Ge) = 106 MPGe
Lifetime MPGe after adding to that 42 gas miles using 1 gallon of gas (1 Ge):
95 miles / (.5 Ge electricity) + (1 Ge gas) = 63.3 MPGe

Note that MPGe is based on the energy content of the fuels (1 Ge = 1gallon of gas = 33.7 kWh of electricity) at the point where they are normally purchased, i.e., at the pump (gas) or from the wall (electricity), not merely when they are used. Gas loses no energy content as it is moved from the pump into the car’s gas tank. Some of the electricity from the wall socket, on the other hand, is used up in the car’s charging circuits. IOW, the Gen 2 Volt’s rating of 53 ev miles per charge and 106 MPGe indicates the car pulls a fixed amount of power from the wall socket (0.5 Ge = 16.85 kWh) and uses X kWh of that in the charging circuits (~2.85 kWh), and Y kWh of that (~14.0 kWh) to propel the car 53 electric miles.
 

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3. MPGe is a "gasoline equivalent" standard for comparing alternative fuel vehicles. For electric cars, it's based on the constant that 1 gallon of gas has the energy equivalent of 33.7 kWh of electricity (as stored energy).

So what this is basically saying is that you would have to get 102 MPG on a car driven by an engine to be as energy efficient as how you drove on electricity since your last full charge.

Edit: In other words, what wordptom said above. :)

2019 Volt LT, Pacific Blue, Power Convenience Package, LT Driver Confidence Package, Comfort Package, nicknamed "Voltemort".
 

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Discussion Starter · #4 ·
Thank you both for the detailed explanations!

Note the Combined MPGe on the left of the display. This is the calculation:
total trip distance / (Ge of electricity used) + (Ge of gas used)

In your screen shot, you’ve used ~0.0357 Ge of electricity and 0 Ge of gas, so ~3.6/0.0357 = ~102 MPGe combined (allowing for rounding off of displayed distance and kWh numbers). Lifetime numbers are similarly calculated... odometer miles / total Ge from the wall + total Ge used. Lifetime MPGe can drop quickly with a small amount of gas use because, as a propulsion fuel, the energy content of electricity is so much greater than the energy content of gas.
So the summary screen showing Lifetime MPGe includes gasoline + electric propulsion, not EV miles only... that explains the low lifetime number. I found an old thread showing how to calculate the approximate miles on ICE using the odometer miles and Lifetime MPG--over 24k miles the previous owner ran about 7k on gas. So he or she was doing considerable gas driving. Whereas I can commute to work and back and still have about 1/3 of the battery charge left.
 

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I don't know about you but those MPGe are meaningless semantics to me. The only thing that matters to me is actual range on battery traveled, KWh used till engine start up and mpg when using only gas engine.
 

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I don't know about you but those MPGe are meaningless semantics to me. The only thing that matters to me is actual range on battery traveled, KWh used till engine start up and mpg when using only gas engine.
Yeah, same to me as well. For my 2018 volt, in winter, I am getting about ~3 miles/kwh and ~5.5 miles/kwh for summer.
My lifetime MPGcs (from voltstats) is 37.1. I can't get that 37.1 from anywhere from the car center display.

If we can strip the electric motor efficiency and look at the efficiency of gas engine only, does volt engine beat the plug-in prius engine?
 

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This may have come up in the above explanations, but it's hard to understand... and after writing all this, it probably still will be. But I figured I'd give it a shot.

One would think that if one drove one mile on gas and one mile on electric, the "Combined MPGe" would be the average of the MPGcs on gas and the MPGe on electricity. So if you drove at 42 MPGcs on gas and 106 MPGe on electric at an even 50/50 ratio, you would think that your lifetime combined MPGe would be 74 MPGe combined.

That is not the case. The expected "combined MPGe" for a Volt user driving exactly 50% of total miles on each energy source would be 60 MPGe combined.

We get this result because the denominator of MPG and MPGe is not miles, but G (gallons) and (gallon equivalents). So for the score to be an average, you have to equalize gallons vs. gallon-equivalents used. A gallon equivalent of electricity is 33.7 kWh. Your Volt has just under a half gallon equivalent of energy stored in the battery, on which it can travel 53 miles. So on a full gallon-equivalent, it would go 106 miles. On a full gallon of gas, it (theoretically) goes 42 miles.

MPGe Combined =
Total Miles Driven_______________________
( Gallons burned + Gallon equivalents consumed)

Expressed differently...

MPGe Combined =
Total Miles Driven_____________________
[ Gallons burned + (kWh consumed/33.7)]

Using the government ratings, you'd consume 238 gallons of gas to drive 10,000 miles. You'd consume 94 gallon-equivalents of electricity to travel the same distance.

What ends up happening is that (based on government ratings) you have to travel about 2.53 (238/94) miles on electricity to balance out 1 mile of gas travel to get to the above-referenced 74 MPGe combined score. In other words, you would have to travel on electricity for 72% of the miles on your car.

As Volt owners, I think we generally tend to overestimate the percentage of miles that we drive our cars on electricity. I'd estimate that at least 90% of the days that I've driven the Volt, I drive on electricity alone for my work commute, errands, etc. But when I use the gas, it's typically on a very long trip where gas miles rack up quickly. So while I perceive that I drive on electricity "90% of the time", my percentage of EV miles lifetime is actually 69.8%, which gets me a lifetime combined MPGe score of 66.2. My score gets dragged down a bit by a northeastern climate.

I view 66.2 as a decent representation of most Volt owners' usage -- full charges each day, commutes entirely on electric, with the occasional long road trip.

So with a lifetime combined MPGe rating of 69.6, it means the previous owner probably drove around 70-75% of the car's miles on EV mode, possibly more if he lived in a colder climate. So he was pretty efficient.

AND...if you don't care too much for math, you can look up the car's lifetime efficiency, miles on each fuel type, and charging stats on the mychevrolet website. Onstar gathers and analyzes information from the car there. While the services needed to update these figures are included for 5 years on each Volt, I don't know how the transfer of ownership of your vehicle would affect your ability to see them. Register for an account and Check out the Charging Stats and History under the Onstar and Connected Services tab.
 

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… We get this result because the denominator of MPG and MPGe is not miles, but G (gallons) and (gallon equivalents). So for the score to be an average, you have to equalize gallons vs. gallon-equivalents used. A gallon equivalent of electricity is 33.7 kWh. Your Volt has just under a half gallon equivalent of energy stored in the battery, on which it can travel 53 miles. So on a full gallon-equivalent, it would go 106 miles. On a full gallon of gas, it (theoretically) goes 42 miles.

MPGe Combined =
Total Miles Driven_______________________
( Gallons burned + Gallon equivalents consumed)

Expressed differently...

MPGe Combined =
Total Miles Driven_____________________
[ Gallons burned + (kWh consumed/33.7)]

...

AND...if you don't care too much for math, you can look up the car's lifetime efficiency, miles on each fuel type, and charging stats on the mychevrolet website. Onstar gathers and analyzes information from the car there. While the services needed to update these figures are included for 5 years on each Volt, I don't know how the transfer of ownership of your vehicle would affect your ability to see them. Register for an account and Check out the Charging Stats and History under the Onstar and Connected Services tab.
It’s very important to keep in mind that the energy content of 1 gallon of gas = the energy content of 33.7 kWh of electricity at the wall plug, and some of the energy content of that electricity is used to charge the battery (= charging losses) before you even have a chance to use it as propulsion fuel. IOW, the quantity of electricity used to drive that 53 ev miles per charge includes not only the kWh Used you see on the usage display, but also the charging losses during the battery charging.

When trying to understand the MPGe numbers you see on the usage display, it helps to think of that 0.5 Ge of gallon equivalents of electric fuel in "one full charge" as approximately equal to ~2.85 kWh of charging losses plus ~14.0 kWh of grid power used from the battery. How far you drive using ~14.0 kWh of battery power is how far you drive using 0.5 Ge, or 16.85 kWh of power from the wall. If your usage display shows, for example, 28 electric miles / 7.0 kWh Used, then 14.0 kWh would take you 56 miles on 0.5 Ge, for a 112 MPGe.

Since 1 gallon of gas = 1 gallon equivalent of gas, it also helps to think of Gallons burned as Gallon equivalents of gas burned, and then it’s easy to add up the total gas + electric gallon equivalents consumed for the combined calculations.

To reword your calculation:

MPGe Combined =
Total Miles Driven_____________________
[ Gallons burned + (kWh consumed from the battery + kWh consumed in the charging circuits/33.7)]

Also note that the MPGe stats available on voltstats.net are not what you will see on the car’s energy display because the site counts all electric miles driven at the EPA-rated MPGe number (e.g., 93, 94, 98, 106) because they cannot read raw kw*hrs from Onstar, so "accurate energy usage could be better or worse than this number." (This is spelled out on the website’s opening page.)
 

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… So the summary screen showing Lifetime MPGe includes gasoline + electric propulsion, not EV miles only... that explains the low lifetime number. I found an old thread showing how to calculate the approximate miles on ICE using the odometer miles and Lifetime MPG--over 24k miles the previous owner ran about 7k on gas. So he or she was doing considerable gas driving. Whereas I can commute to work and back and still have about 1/3 of the battery charge left.
Math is a useful tool when dealing with your Volt’s statistics.

Odometer reading / Lifetime MPG reading = lifetime gallons of gas used

(the more precise the MPG number, the better the results, and if you register your Volt on voltstats.net, their website display shows 2 decimal places, and the downloadable data is 4 decimal places).

What is interesting about that lifetime gas used number is that it is also the number of ev miles you need to drive without using any gas that will get your lifetime MPG number to increase by 1.0000. For example, that website shows my 2012 Volt’s lifetime MPG is 151.08 and my odometer reads 49939.47. Thus my lifetime gas consumption is 330.55 gallons. If I now put 330.55 more electric miles on the odometer without any gas miles, my lifetime MPG will increase to 152.08 MPG.

One’s ev % depends a lot on the driving habits. Around town I rarely drive beyond a half-charge’s worth of distance on any day (~99% ev), but I do take a long road trip (500+ miles) most summers, including two 4,000+ mile trips from Oregon to Michigan and back. Long road trips can quickly put a dent in the lifetime MPG and MPGe because the gas tank holds so much more fuel than the electric fuel tank. Gas is a low mileage propulsion fuel because so much of the energy content is converted into heat, but on such a trip it is easier to use the low-mileage fuel you have than to take the time to stop and refuel the high-mileage fuel.

The Gen 2 Volt’s electric fuel tank - the battery - holds only a half-gallon of energy equivalent electric fuel (0.5 Ge) and nearly 9 Ge of energy equivalent gas fuel. Once you charge the battery and unplug from the wall, you can drive, at window sticker ratings, 53 electric miles and ~9 x 42 = ~378 gas miles. During the first 53 miles of that road trip, you use only a half-gallon of Ge fuel (106 MPGe), but you then burn 1 gallon of Ge fuel for each additional 42 miles you continue that road trip. By the time you reach mile ~431, you’ve consumed a total of ~9.5 gallons of energy equivalent fuel, and 431/9.5 = 45.4 MPGe. Your trip MPG, by the by, has gone from infinite (250+) for the first 53 miles to 47.9 at the end, an even more drastic drop, the result of a single long road trip. The impact on lifetime numbers will vary, depending on the pre-trip lifetime gas used numbers.

Here’s something else to consider about MPGe. MPGe is not much more meaningful than MPG, except as a slightly better relative gauge of your driving efficiency. MPGe ignores regen. Note that the Volt’s regenerative braking system creates electricity and puts it into the battery as you drive, and the energy content of the regen is then used by the motor. Surely regen has the same "energy content" as "from the wall" electricity and could be included as electric fuel consumption in the MPGe calculations. Unknown is the regen "charging losses" (i.e., are there any charging losses when regen is "pulled from the MGB generator output" and put into the battery?).

The "energy content" of regen is not counted as "fuel" when dividing the total electric miles you drive by that 0.5 Ge in a full charge. The more downhill regen you can create and use on a trip, the more "electric miles without using fuel" you can add to the total ev miles included in the MPGe calculation... and these fuel-less electric miles also count toward your "lifetime MPGe." (Of course, regen obtained and used while driving in Extended Range Mode counts as Gas Miles, increasing the total Gas Miles but not adding to Gas Used, so regen battery power also increases your MPGcs gas mileage.)
 

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We've been over this before, but the MPGe reading on the Volt does not ignore energy created by regen braking. If you use the paddle or simply coast down a long hill, you will see the Volt's kWh consumed since last charge roll backwards, and if you've driven less than a full charge, you can usually see fairly significant increases in the reported efficiency MPGe score, as your miles driven increases while your energy consumed is reduced. I don't know how regen could possibly be better accounted for.
 

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We've been over this before, but the MPGe reading on the Volt does not ignore energy created by regen braking. If you use the paddle or simply coast down a long hill, you will see the Volt's kWh consumed since last charge roll backwards, and if you've driven less than a full charge, you can usually see fairly significant increases in the reported efficiency MPGe score, as your miles driven increases while your energy consumed is reduced. I don't know how regen could possibly be better accounted for.
Sure, the mileage will go up if the rate of fuel consumption goes down, as it will when driving downhill, but I’m not sure how the total quantity of "energy consumed is reduced" during a drive by driving downhill...

At the top of the long hill, my display is showing, let’s say: 7 kWh Used. That means my Gen 2 Volt now has ~7 kWh usable left in the battery from the ~14.0 kWh of usable power in a full charge.

At the bottom of the hill, because regen created while driving is subtracted from the kWh used display, my display has been rolled back to show 6.5 kWh Used. That means my Gen 2 Volt now has ~7.5 kWh of usable power left in the battery.

I drive until the car switches to gas, and the display shows 14.0 kWh Used.

My motor used 7 kWh from the battery to reach the top of the hill. After reaching the bottom of the hill, the motor used an additional 7.5 kWh from the battery before the car switched to gas. That’s 14.5 kWh of power used for propulsion, but the MPGe calculations use only the 14.0 kWh of grid power stored in the battery and ignore the 0.5 kWh of regen.

Note that if you drove the same downhill in Hold Mode or with a fully depleted battery, you would still get the 0.5 kWh of regen, and would drive the same battery-powered distance using that regen when you reached level terrain, and that distance would add to your Gas Miles total and your Gas MPGe (or Gas MPGcs) would not include this fraction of a Ge of regen in the total fuel consumption used to drive the total Gas Miles.
 

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All this reminds me of looking at all the twists and turns of yarn in a sweater when all we want is to put something warm on to keep out the cold. Full disclosure, I have taken a double major in math and physical chemistry in university although I did take an honours math class where I didn't know what the hell they were talking about. It was an extra class so I was able to drop it with no effect to my standing (whew)!
 

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Sure, the mileage will go up if the rate of fuel consumption goes down, as it will when driving downhill, but I’m not sure how the total quantity of "energy consumed is reduced" during a drive by driving downhill...

At the top of the long hill, my display is showing, let’s say: 7 kWh Used. That means my Gen 2 Volt now has ~7 kWh usable left in the battery from the ~14.0 kWh of usable power in a full charge.

At the bottom of the hill, because regen created while driving is subtracted from the kWh used display, my display has been rolled back to show 6.5 kWh Used. That means my Gen 2 Volt now has ~7.5 kWh of usable power left in the battery.

I drive until the car switches to gas, and the display shows 14.0 kWh Used.

My motor used 7 kWh from the battery to reach the top of the hill. After reaching the bottom of the hill, the motor used an additional 7.5 kWh from the battery before the car switched to gas. That’s 14.5 kWh of power used for propulsion, but the MPGe calculations use only the 14.0 kWh of grid power stored in the battery and ignore the 0.5 kWh of regen.

Note that if you drove the same downhill in Hold Mode or with a fully depleted battery, you would still get the 0.5 kWh of regen, and would drive the same battery-powered distance using that regen when you reached level terrain, and that distance would add to your Gas Miles total and your Gas MPGe (or Gas MPGcs) would not include this fraction of a Ge of regen in the total fuel consumption used to drive the total Gas Miles.
Your quantity of energy delivered from the wall used is effectively reduced by the amount of energy regenerated.

I suppose there could be a separate line for energy produced by regeneration, but reducing the total consumption seems the only way to appropriately account for the efficiency impact of regenerative breaking.

Merely adding regenerated electricity as additional energy consumed would have the effect of making the car appear less efficient, which makes no sense.

The scores after the battery is depleted are akin to the MPG scores you would get from a non-plug-in hybrid by use of regenerative braking, so I don't know how that's a bad thing.
 

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Your quantity of energy delivered from the wall used is effectively reduced by the amount of energy regenerated.
The Volt’s energy usage kWh Used display shows a net calculation, grid power used less regen created, not a meter reading of total kWh used. When you drive far enough to deplete the Gen 2 battery of "one full charge," the total electric energy you have consumed is the 0.5 Ge of fuel (16.85 kWh) you pulled from the wall to charge the battery plus the uncounted Ge (kWh) of regen you created and used while you drove.

I suppose there could be a separate line for energy produced by regeneration, but reducing the total consumption seems the only way to appropriately account for the efficiency impact of regenerative breaking.

Merely adding regenerated electricity as additional energy consumed would have the effect of making the car appear less efficient, which makes no sense.
The regenerative braking system recharges the battery with electricity that can then be used by the motors until the battery is again recharged from the wall. This system doesn’t increase the efficiency at which the motors use electricity, it merely creates more electricity for them to use. Why would you not count it as fuel when calculating mileage?

If you reach the crest of a long hill and start down just before the battery is depleted, then put enough regen in the battery to recharge it fully by the time you reach the bottom, and then drive until the battery is empty, regenerative braking has not "doubled the efficiency" of the car simply because the car has been driven downhill, even if you end up doubling the distance you normally drive on a full charge. That 212+ MPGe you may see on the Gen 2 Volt energy usage display after such a drive is not because the car is more efficient in hilly terrain, it’s because half of the electric fuel used to drive those 106+ electric miles was created by the car’s generator, not pulled from the wall plug.
 

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My solution: a Chevy Bolt, where I never think about gas, not even it's MPGe gas rating, haha.

MPGe seems like it's the "horsepower" route for people whose normal frame of reference is how many horses are moving their carriage. In this case, "How much would that be in miles per gallon of gas?" Like horsepower, eventually the original measurement—the pulling power of a horse—is meaningless for most people and it just becomes a comparison number.

Like the yarn sweater example above, all I care about is how far can I drive the car on a full battery. So I suppose MPGe city and MPGe highway can give some added guidance, but meh, not something I look at.
 

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I shopped for a Gen 1 Volt and finally purchased a like new black 2014 with 60,000 miles for $8995. It has a brand new battery, and a 12 month, 12,000 mile GM factory warranty (due to the CA lemon law). I am amazed how well engineered it is and the quality of everything in the car.

One thing I noticed while shopping was that most all of the many Volts I looked at had lifetime MPG values of from 40-45 MPG. This indicates that the owners used mostly gas. It also indicates that from an environmental standpoint the Volt fleet is a failure compared to Prius's. The Prius gets 50 MPG and uses no electricity. All of the Volts I saw that had 40-45 MPG lifetime also likely used some electricity during their lives which isn't even included in the MPG value. So from a lifecycle CO2 emission standpoint, as a fleet, the Volts I saw probably generated 20 to 30% more CO2 than hybrid Prii.

Over the first 1000 miles, my driving and frequent charging has resulted in 152 MPG but I am concientious about only driving on electric. What is your lifetime MPG value for your Volt? Is 40-45 MPG typical? or is something higher thatn is at least competetive with Prii, like 60-70 MPG lifetime more typical for our Volt fleet? Do only enthusiasts who focus on EV driving do better than Prii, and are most Volts bought primarily to be able to drive in the Carpool/Hybrid lane? Why do owners give up on charging? Are their commutes a hundred miles? Are they lazy about charging? Do they take long weekend trips on gas that overwhelm the EV driving miles?
 
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