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Where exactly does this number come from . . . . and what affects it? I thought I knew, but . . . .

Bought our '17 Premiere about 8 months ago with about 20K on it. The Lifetime number was only 55 point something, which I thought was awfully low - Even if it had been driven using the ICE 90% of the time, I would have expected it to be higher than that

Drove it home 1,000 miles largely using the ICE and if the number changed at all, it was only a tenth or two - I didn't really notice. Over the next several months and 2,500 or so miles, 90% or more of which were all electric miles, the number slowly crept up to about 57.5 - I regularly see MPGe numbers around 100 or better when driving the car

Then one day out of the blue, the Lifetime MPGe number all at once reset to 50.0 and stayed there for 2 or 3 weeks. Now it's reset again to 52.0 and hasn't changed at all the last several times I've driven it. Mileage is now a little over 25,000 and 90 or 95% of the last 4,000 miles have been electric and the worst MPGe I think I've ever seen has been in the high 80's . . . . but the Lifetime MPGe is worse than when I bought the car!

Whats really strange is the overnight reset from 57.5 down to 50 . . . . and then another reset from 50 to 52. Anyone have any idea as to what I'm seeing and what it means?

Don
 

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My Lifetime MPGe has never reset as you described. It slowly increases while driving on electric then drops down sharply when on the ICE. Mine is currently at 73.84 MPGe after 26,700 total miles, 20,000 were electric. The lowest that it has dropped to is about 66, but that was during my first long road trip (700 miles) on gas. Every year, during the same road trip, it drops again but not as far as the previous year because it is a continuous "lifetime" average. My last long road trip, it dropped to about 71.4 MPGe, now it is at 73.84 and increasing, slowly every day.
You can check other Volts on VoltStats.net even if you don't have an account of your own. Here is a link to mine:
Select "Reading History" then Graph Type MPGe, then "Update".

https://www.voltstats.net/Stats/Details/7685
 

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On thing that will cause the ICE to start on it's own without your knowledge and drop your lifetime MPG number rapidly is if the settings for "Engine Assist Heat" is not on "deferred". This happened to me this winter and I couldn't figure out why my ICE was starting and my lifetime MPG was falling when I still had battery left. Someone on this forum pointed out this setting to me.
Today my lifetime MPG is at 133 mpg. Running the ICE makes it drop quickly, and using battery only makes it rise very, very slowly.
 

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The trick here is thinking about what combined MPGe is measuring. It's measuring how far you go on a "gallon-equivalent" of energy. For gas, that's easy, it's measuring how far you go on a gallon. For electric, it's measuring how far you go on 33.7 kWh of electricity. If you use the Volt's EPA ratings, which we'll do to reduce the argument over proper figures, you'd get 42 miles per gallon of gas and 106 miles per gallon equivalent of electricity.

Now, the critical thing to think about here is what the denominator of the fraction is. It's per "gallon equivalent". The car doesn't average your MPGe rating and MPG rating based on how many miles you've driven in each mode -- it does so based on how much energy you've consumed.

What this means is that gas miles have a significantly higher impact on the MPGe combined score.

As an illustration, let's assume you drive 106 miles in a day. For the first 53 miles on the battery, you get the Volt's EPA-rated 106 MPGe. For the next 53 miles, you get the Volt's EPA rated 42 MPGcs. It's tempting, and intuitive, to want to figure out your combined MPGe by just averaging those two numbers, as you drove an equivalent amount of miles on each fuel source. So you'd think that your combined MPGe was 74, because (106 + 42)/2=74. But it isn't. What you have to consider is total amount of gallon equivalents used.

So driving on electric, you used a half-gallon-equivalent of energy to travel 53 miles. On gas, you used 1.26 gallons to travel the same distance. So your MPGe combined equation looks like this

Gas Miles (53) + Electric Miles (53) = 106 miles/1.76 Gallon-eq. = 60.2 MPGe combined
Gallons (1.26) + Gallon-equiv (0.5)

What this really means is that, assuming figures close to the government ratings, every mile you travel on gas will impact your combined fuel economy score 2.5x negatively as much as a mile traveled on electricity will impact it positively. So gas usage will show noticeable decreases to the score quickly, while electric usage will show small increases over much longer periods of time.

The lifetime number you reported from when you bought the car would indicate to me that the Volt was driven about 55% of its miles on gas, and about 45% on electric. Even if you have driven it 90% in electric for the next 4000 miles, you've only nudged the lifetime EV percentage up to about 60%.

All that said, the sudden drops do seem a little off. However, if you've had cold mornings recently, as someone else mentioned, that will also significantly impact your score very quickly, as the ERDTT settings not only run the engine, but run it constantly until the coolant comes up to temp. I've regularly gotten 25mpg-cs or less for the 5 miles or so of my commute that the Volt requires the engine to run. My Volt was at 70.3 MPGe combined before the recent cold spell, which has forced my Volt to run the engine for heat about 70% of the time in the morning recently. Despite still driving 80% of my commute on electric (averaging about 85 MPGe in the cold temps), my lifetime score has dropped to 69.6 MPGe because I use up about three tenths of a gallon of gas each day and the remaining chunk of my 28 mile commute only uses ~9 kWh, which is about a third of a gallon-equivalent.

I have noticed that the Lifetime combined number seems to adjust at somewhat odd intervals. I don't know whether it has to reach a certain incremental change from the stated figure to change, or if it only updates every few weeks, but when mine changed, it did not drop 0.1 as a time, as you can notice with the "since last charge" numbers. Instead, like you said, I left it one day at 70.1 and came back the next and it was 69.6 at startup.
 

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Discussion Starter #5
What this really means is that, assuming figures close to the government ratings, every mile you travel on gas will impact your combined fuel economy score 2.5x negatively as much as a mile traveled on electricity will impact it positively. So gas usage will show noticeable decreases to the score quickly, while electric usage will show small increases over much longer periods of time.
Excellent explanation . . . . and thank you for that! Really appreciated

So, if I drive it 2.5 miles EV for every single ICE mile, the Lifetime number should stay about the same - Little if any increase or decrease

From the time I had roughly 57.5 showing, I know I've driven it about 10 EV mles for the few ICE miles I've used - Zero ICE miles except for when it tells me I need to run the engine and then only for 8 or 10 miles. Over the past 4,000 miles, we've never ever run the battery down completely to where the engine starts - Not once. So . . . . my 57.5 should still be slowly increasing, but something reset it overnight to 50.0 - Not 49.9 or 50.1, but 50.0. And then a week or two later it reset to 52.0 and it has stayed there

Is this number generated wholly within the car, or does OnStar have something to do with it? The car isn't driven that often and when it's not on the road it's in a garage where OnStar and GPS aren't accessible

Don
 

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Easy to forget that MPGe is not MPG or Total Miles (EV+ICE) per Gallon consumed.
 

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Excellent explanation . . . . and thank you for that! Really appreciated

So, if I drive it 2.5 miles EV for every single ICE mile, the Lifetime number should stay about the same - Little if any increase or decrease

From the time I had roughly 57.5 showing, I know I've driven it about 10 EV mles for the few ICE miles I've used - Zero ICE miles except for when it tells me I need to run the engine and then only for 8 or 10 miles. Over the past 4,000 miles, we've never ever run the battery down completely to where the engine starts - Not once. So . . . . my 57.5 should still be slowly increasing, but something reset it overnight to 50.0 - Not 49.9 or 50.1, but 50.0. And then a week or two later it reset to 52.0 and it has stayed there

Is this number generated wholly within the car, or does OnStar have something to do with it? The car isn't driven that often and when it's not on the road it's in a garage where OnStar and GPS aren't accessible

Don
I'm not sure of the interaction with OnStar. What I have observed is it doesn't seem to adjust completely live but seemingly recalculates each day or at some kind of time interval.

I haven't completely thought it through, and a lot of "Voltmath" is not the most intuitive, but I would think that, yes, every 2.5 EV miles should increase your score about as much as every gas mile decreases it. This, I think, assumes that you're starting at about 74 MPGe combined (halfway between 106 MPGe and 42MPGcs). If you're closer to 42 for the lifetime combined number, gas miles will show a smaller impact on your score. I kind of think about it like baseball averages. It's about distance from starting point. So while a .300 hitter that goes 1 for 3 will decrease his average twice and increase it once, his net average shows a gain.

I'm not 100% sure on 74MPGe as the ideal starting point... It may be more like 60 MPGe combined (what one would expect for exact 50/50 EV/gas usage). I need to think about that.

What was your reported gas MPG when the engine was forced on for the 8 or 10 miles? That will also impact it. I get around 42 mpg or more only when I have longer highway cruises and the engine gets a chance to come up to temp. If it's just the last few miles of my drive, or if I have to do a lot of stop and go, or if the engine comes on during preheat or the first few miles, my gas MPG can drop into the 20s or even teens for the 8+ miles I have to use the ICE for each way, and despite getting about 85MPGe for the rest of my trip on electric, I'll get something in the range of 48MPGe combined for the whole day.

I've also been talking about all of this as if you're discussing the combined MPGe number in your infotainment screen -- is that the case? Or is this a number reported by OnStar through the MyChevrolet app or the VoltStats platform? I canceled my Onstar subscription after the 3 free months, so I'm not sure how those numbers are calculated.
 

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I guess I have a had a different impression of what the lifetime gpm display means. I thought it was the total miles the car has been driven divided by the number of gallons of gas the car has used. That simple, all done within the car's computer, nothing to do with ONSTAR. I realize that is a bit of a phony number, as far as hydrocarbon used to power your car, but does have significance. The higher the number the less gasoline you use to get there. In my case, that math would closely relate to what has happened since I bought the car. If I take the gas I bought since buying the car new divided into my total miles, it comes out very close. As electricity is cheap where I live, and it is all hydroelectrically produced, in my case, I feel the number is fairly accurate.
 

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I guess I have a had a different impression of what the lifetime gpm display means. I thought it was the total miles the car has been driven divided by the number of gallons of gas the car has used. That simple, all done within the car's computer, nothing to do with ONSTAR. I realize that is a bit of a phony number, as far as hydrocarbon used to power your car, but does have significance. The higher the number the less gasoline you use to get there. In my case, that math would closely relate to what has happened since I bought the car. If I take the gas I bought since buying the car new divided into my total miles, it comes out very close. As electricity is cheap where I live, and it is all hydroelectrically produced, in my case, I feel the number is fairly accurate.
You're correct for the MPG rating. What we were talking about is "Combined MPGe" which is a somewhat useful measure that takes into account how efficiently you drive even on electric -- you can get to it by clicking on the MPG box on the main energy display. The combined lifetime MPGe shows up just below the box.
 

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Discussion Starter #11
What was your reported gas MPG when the engine was forced on for the 8 or 10 miles? That will also impact it.
Those miles were done at 60 or 65 mph - Reported mpg was very high . . . 45 or 46

I've also been talking about all of this as if you're discussing the combined MPGe number in your infotainment screen -- is that the case? Or is this a number reported by OnStar through the MyChevrolet app or the VoltStats platform? I canceled my Onstar subscription after the 3 free months, so I'm not sure how those numbers are calculated.
Yes, it's all from the info screen. I've never used OnStar for anything, nor the MyChevrolet.app and have never looked at Volt Stats. Would the latter show me anything about the previous owner's usage??

Don
 

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I guess I have a had a different impression of what the lifetime gpm display means. I thought it was the total miles the car has been driven divided by the number of gallons of gas the car has used. That simple, all done within the car's computer, nothing to do with ONSTAR. I realize that is a bit of a phony number, as far as hydrocarbon used to power your car, but does have significance. The higher the number the less gasoline you use to get there. In my case, that math would closely relate to what has happened since I bought the car. If I take the gas I bought since buying the car new divided into my total miles, it comes out very close. As electricity is cheap where I live, and it is all hydroelectrically produced, in my case, I feel the number is fairly accurate.
Let’s be clear that MPG, MPGcs, and MPGe represent different values.

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

MPGcs = fuel 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 of gas = 1 gallon of gas, and 1 Ge of electricity = 33.7 kWh of "from the wall plug" electricity.
 

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My Lifetime MPGe has never reset as you described. It slowly increases while driving on electric then drops down sharply when on the ICE. Mine is currently at 73.84 MPGe after 26,700 total miles, 20,000 were electric. The lowest that it has dropped to is about 66, but that was during my first long road trip (700 miles) on gas. Every year, during the same road trip, it drops again but not as far as the previous year because it is a continuous "lifetime" average. My last long road trip, it dropped to about 71.4 MPGe, now it is at 73.84 and increasing, slowly every day.
You can check other Volts on VoltStats.net even if you don't have an account of your own...
Note that the MPGe numbers appearing in Voltstats.net are estimates, and not based on actual energy usage. As the site administrator writes, "This is just an estimation - I cannot currently read raw kw*hrs from Onstar, so actual energy usage could be better or worse than this number." The formula used on that site is: Total Miles / (Gallons Burned + (EV Miles / EPA rating)). The actual formula is: Total Miles / (Ge of gas used + Ge of electricity used)
 

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Where exactly does this number come from . . . . and what affects it? I thought I knew, but . . . .

Bought our '17 Premiere about 8 months ago with about 20K on it. The Lifetime number was only 55 point something, which I thought was awfully low - Even if it had been driven using the ICE 90% of the time, I would have expected it to be higher than that

Drove it home 1,000 miles largely using the ICE and if the number changed at all, it was only a tenth or two - I didn't really notice. Over the next several months and 2,500 or so miles, 90% or more of which were all electric miles, the number slowly crept up to about 57.5 - I regularly see MPGe numbers around 100 or better when driving the car

Then one day out of the blue, the Lifetime MPGe number all at once reset to 50.0 and stayed there for 2 or 3 weeks. Now it's reset again to 52.0 and hasn't changed at all the last several times I've driven it. Mileage is now a little over 25,000 and 90 or 95% of the last 4,000 miles have been electric and the worst MPGe I think I've ever seen has been in the high 80's . . . . but the Lifetime MPGe is worse than when I bought the car!

Whats really strange is the overnight reset from 57.5 down to 50 . . . . and then another reset from 50 to 52. Anyone have any idea as to what I'm seeing and what it means?

Don
Consider this possible scenario. The original owner of your 2017 Volt had purchased the car to drive his five day a week, 134.9-mile round-trip commute, and drove it for 30 weeks (150 commutes). Each day he used up the full charge, driving the rated 53 ev miles on the way to work, and then drove the rest of the way to work, and all the way home again, on gas (81.9 gas miles, 1.95 gallons @ 42 mpg). He then realized that a Bolt would allow him to make the commute on electricity only with no range anxiety, so he bought a Bolt and sold the Volt, which you then bought.

On each of those 150 days of driving, he used one half gallon of "energy equivalent" electric fuel (i.e., the 0.5 Ge in one full charge) and 1.95 gallons of "energy equivalent" gas fuel (i.e., 1.95 gallons of gas = 1.95 Ge of gas), so his daily "combined MPGe" was 134.9 miles/2.45 Ge = 55.1 MPGe.

During those 150 days, he drove (150 x 134.9=) 20,235 miles, and used ~292.5 gallons of gas (=292.5 Ge of "energy equivalent" gas fuel) and 150 full charges (@ 0.5 Ge per charge = 75 Ge of "energy equivalent" electric fuel).

20,235 miles/(292.5 + 75) Ge = 20,235/367.5 = 55.1 MPGe.

Of those 20,235 miles, 7,950 were ev miles (39.3%) and 12,285 were gas (60.7%). MPG = 69.2

Then you drove it home, adding ~1,000 gas miles to the existing ~20K miles and ~23.8 Ge to the lifetime Ge fuel consumption.

As you arrived home with your Volt, the Lifetime MPGe = 21,235 miles/391.3 Ge = 54.3, i.e., not much of a drop from 55.1.

You then started using the car, and added an additional 2,500 miles... let’s say 3.3% gas (84 miles = 2 Ge) and 2,416 miles on battery (/106= 22.8 Ge electricity)

At that point the Lifetime MPGe = 23,735 miles/416.1 Ge= 57.0

You then added another ~1,500 miles... let’s say 9.8% gas (147 miles = 3.5 Ge gas) and 1,353 miles on battery (/106 = 12.8 Ge electricity).

Lots of ev driving, not much gas, but still, Lifetime = 25,235 miles/432.4 Ge = ~ 58.4 MPGe

I don’t know what’s happening with the overnight resets of your Volt’s Lifetime MPGe. It sounds like a glitch.

As for the other issues, as long as you are driving reasonably efficiently in Electric Mode, you should regularly see trip ev MPGe numbers of 100 or better when driving the car (window sticker rating is 106 MPGe). That number is related to your driving that day, not to a Lifetime number. Even when gas is used, the trip ev MPGe applies only to the electric miles. The combined MPGe at the top left of the display will start to decrease once you start using gas during a trip.

Your Volt’s current lifetime fuel consumption has about three times as much Ge gas consumption as Ge electric consumption, and because the distance you need to drive to use 1 Ge of gas is so far (106 miles at window sticker ratings), it takes a lot of gas-less ev-only driving to increase the Lifetime MPGe number. Do you really care, as long as the trip ev MPGe is showing that you are reasonably efficient at driving your Volt in Electric Mode?
 

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You're correct for the MPG rating. What we were talking about is "Combined MPGe" which is a somewhat useful measure that takes into account how efficiently you drive even on electric -- you can get to it by clicking on the MPG box on the main energy display. The combined lifetime MPGe shows up just below the box.
Wow, I have never looked at that. Never even knew it was there. I am not sure I will ever pay much attention to it either. Maybe I am wrong, but I have always thought of MPGe as an artificial, non-scientific, bureaucratically developed number used to try to compare Apples to Oranges. Sort of ambiguous. If you wanted to evaluate how efficiently you drive on electricity from one time to the next, I would think KW/mile might be a better number. To directly compare electrical use to gasoline use gets tricky as you are forced to make assumptions that aren't necessarily true and/or consistent. It is seemingly a number that could easily be manipulated one way or the other.
 

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Wow, I have never looked at that. Never even knew it was there. I am not sure I will ever pay much attention to it either. Maybe I am wrong, but I have always thought of MPGe as an artificial, non-scientific, bureaucratically developed number used to try to compare Apples to Oranges. Sort of ambiguous. If you wanted to evaluate how efficiently you drive on electricity from one time to the next, I would think KW/mile might be a better number. To directly compare electrical use to gasoline use gets tricky as you are forced to make assumptions that aren't necessarily true and/or consistent. It is seemingly a number that could easily be manipulated one way or the other.
You're correct that it was bureacratically created, but it is quite scientific and not artificial. Basically, they created the measure to avoid all of the inconsistencies you're alluding to. MPGe is just a conversion of miles per kWh into a different fraction -- Miles per 33.7 kWh.

The EPA determined there is 33.7 kWh of energy in a gallon of gas. For gas, a large portion of that is converted into heat in combustion while the remainder is converted into kinetic energy that actuates the various engine components and turns axles to propel the car forward. That's why even the most efficient gas vehicles travel much shorter distances on a gallon of gas than electric vehicles travel on 33.7kWh of energy -- because gas vehicles create much more waste heat than electric.

You're correct that there are problems with the comparison of gasoline efficiency and electric efficiency, but those are mostly caused by misconceptions about what the number represents. It does not purport to represent a monetary equivalent between what you'd spend on gasoline vs. electricity -- those numbers are far too variable both over time and geographically to make up the basis for electric vehicle efficiency ratings. (Think about having to constantly adjust for the price of gas and the price of electricity -- my gas prices have fluctuated up and down by 20% over just the past year, much less the past 5 years) They also do not purport to represent an ecological impact comparison, which can significantly vary based on local power infrastructure and fuel sources.

What they do is make a somewhat meaningful means of comparison of efficiency between electric vehicles in a language and scale that's similar to what people ordinarily associate with fuel efficiency. If you're cross-shopping a Model S and a Model 3, and someone tells you that you can go 2.91 miles per kWh in the Model S, but you can go 3.85 miles per kWh in the Model 3, the customer is pretty likely to roll their eyes, think what a nerd, and really have very little idea of what it means. When you say that the Model S is rated at 99 MPGe and the Model 3 is rated at 130 MPGe, we can observe a significant difference that we can more easily process. It's very clear in that scenario that the Model 3 is about 30% more efficient than the Model S. While the math is the same on the miles per kWh scale, the message is not as clear or familiar to us.
 

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So am I correct to gather from what you have said that your MPGe is derived on a mathematical formula that compares how much you drive on gas versus how much you drive on electricity based on the vehicle's EPA ratings for that vehicle on both gas and electricity, reporting in a number that people can more easily relate, but doesn't really take into consideration how efficiently you actually drive?
 

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So am I correct to gather from what you have said that your MPGe is derived on a mathematical formula that compares how much you drive on gas versus how much you drive on electricity based on the vehicle's EPA ratings for that vehicle on both gas and electricity, reporting in a number that people can more easily relate, but doesn't really take into consideration how efficiently you actually drive?
You're kinda sorta correct. The EPA MPGe number is how efficient the car has been tested to be on the EPA's test cycle. For the Volt it's rated at 106 MPGe. This is for battery powered EV use only. For the Volt, it's rated at 106 MPGe combined. Think of this as the equivalent of what you see in commercials for regular cars reporting 40 MPG ratings and the like. It's what the government has objectively tested the car to consume under a specific set of circumstances.

The MPGe number reported in the upper battery portion on the Energy Screen of the Infotainment system reports your MPGe score (which is solely based on your miles per kWh used) since last charge.

The MPGe combined lifetime number represents the combined efficiency of electric and gas usage. And yes, it is arrived at by a mathematical formula, but I'm not really sure how it's subject to manipulation.

I find the MPGe combined number to be fairly useful compared to the straight lifetime MPG number because for me and a lot of Volt drivers, that number just stays at 250+ at all times, whereas the MPGe combined number gives me a better idea of how my driving during a particular time period compares to the lifetime efficiency of using the car.
 

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Thanks for educating me, Mjones. Perhaps the bottom line is not to worry too much about any of the numbers. What I pay attention to most is maximizing the electric miles and minimizing the ICE miles every time I go somewhere. Perhaps both Lifetime MPG and MPGe are more "feel good" numbers than an accurate measurement of how "well" you are doing, whatever "well" means.

Talk about "feel good" number, Joereal, you have to feel good. Probably have never really needed an oil change on your ICE.
 
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