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Discussion Starter · #1 ·
I've only had my 2011 Volt for about 1/2 a year, so I am still geeking out a little bit over tracking fuel burned, MPG, etc... Likely not to the same extent as some others, but I do track it a bit.

I have a spreadsheet with (among other things...) the odometer reading and lifetime MPG. I figured that if I divide the odometer by the lifetime MPG (mile / [mile/gal]) I would get the number of gallons the car has burned over it's lifetime. Pretty simple. Here is the result over time:
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I am puzzled by the result... How is it possible that the calculation suggests that my gallons consumed has decreased since late July? I have a suspicion that I know the answer (i.e. the lifetime MPG was reset at some point), but I wanted to see if anyone else has tracked their data similarly and has seen a similar "error" in the calculated lifetime gallons burned.

Get out you calculator watches! :p
MikeB
 

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Yes, it does appear your center console Lifetime MPG meter was reset by the service department at some time (I seem to recall there’s also now an app that will let you do that yourself). Possibly the person who sold the Volt thought the reading would discourage purchasers and had it reset (meaning, it was perhaps reset around odometer reading 58,800, so if you adjust the odometer reading to read the distances the car has been driven since that possible reset point, it suggests you’ve used about 66 gallons of gas in the last 6 months while driving those ~5,000 miles)...

Don’t know where you could get an accurate reading of the real Lifetime MPG. My understanding is the MPG reading in the MyChevy app is a reading of the number showing on the console display, not of the calculated number (total miles / total gas consumption, numbers that the computer does keep in the database). If you have an OnStar account that gives you the status on the mychevy app showing total miles and total ev miles, you could use those numbers to calculate the total gas miles on the car. Then divide that by the window sticker rating of 37 mpg (or use the MPGcs you actually achieve according to the energy usage display) to get an estimate of the car’s total lifetime gas consumption.

...and then use THAT lifetime consumption number with the current odometer reading to get a Lifetime MPG number for the vehicle.
 

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The problem is the average "lifetime MPG" number is essentially meaningless. It's calculated by using traditional MPG (miles per gallon of gas burned) and MPGe (miles driven on electricity divided by the number of kWh used and a conversion factor to convert the energy equivilent of a kWh to a gallon of gas).
 

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Discussion Starter · #4 ·
Yes, it does appear your center console Lifetime MPG meter was reset by the service department at some time
Tom,
I have never looked into the app and do not have an OnStar acct. Since the cell service is going away on older Volts, I assume I will never have this data...

However, I do have a copy of the title from the fellow I purchased the car from... as I had previously mentioned in other threads, I had bought this car with a salvage title ~8 months ago (as had the fellow I had purchased it from - he owned it for 5 or 6 years). The mileage on his salvage title showed 29,896 miles. I would bet that when the car was put back together, someone made some programming changes which including zeroing out the lifetime MPG. When I subtract that number of miles from the current odometer reading, the calculated number of gallons now completely makes sense. In fact I see that the few fill-ups data points I have also seems to fall into line (or is pretty close).

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I was surprised how quickly the lifetime MPG was increasing for the small number of miles I had driven the car, but now I know why!
Mystery solved. We can all put our calculator watches back in the vault.
MikeB


 

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The problem is the average "lifetime MPG" number is essentially meaningless. It's calculated by using traditional MPG (miles per gallon of gas burned) and MPGe (miles driven on electricity divided by the number of kWh used and a conversion factor to convert the energy equivilent of a kWh to a gallon of gas).
Anything is meaningless if you don't have a use for it. It's excellent, for example, to make valuable guess as to how many of the odometer miles were gasoline-fired versus on battery.
 

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Discussion Starter · #6 ·
RIGHT!?
 

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The problem is the average "lifetime MPG" number is essentially meaningless. It's calculated by using traditional MPG (miles per gallon of gas burned) and MPGe (miles driven on electricity divided by the number of kWh used and a conversion factor to convert the energy equivilent of a kWh to a gallon of gas).
The "lifetime" MPG has little meaning for a car where use of gas is optional, but it is rooted in reality and is not related to MPGe at all. The total miles on the car divided by the car’s lifetime MPG gives you the "lifetime" gas used, in gallons (doesn’t work if the MPG has been reset). And using the window sticker mpg rating or, better, your own Volt’s "gas mileage" number you calculate from recent gas miles /gas used (best if you drive far enough to use 1 gallon or more), you can multiply that "lifetime" gas used number by the gas mileage estimate to obtain an estimate of lifetime Gas Miles. Using the odometer reading then, math gives you the total ev/gas miles breakdown.

The "lifetime" number of gallons of gas used, by the way, is also the number of ev miles you need to drive without using any gas to have the MPG increase by 1.000.

The MPGe number, on the other hand, includes only the kWh of electricity put into the battery from the wall plug. The energy content of regen isn’t counted as electric fuel in the MPGe energy-equivalent mileage calculations. Any Volt driver who spends time driving up and down hills will accumulate downhill regen powered distances (doesn’t get counted as electric fuel used) and downhill distances propelled by gravity, not by electricity. A "mileage" number that doesn’t include all the fuel used to drive a recorded distance is a meaningless number, especially when distances achieved without using any fuel at all are included in the total.
 

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The "lifetime" MPG has little meaning for a car where use of gas is optional, but it is rooted in reality and is not related to MPGe at all. The total miles on the car divided by the car’s lifetime MPG gives you the "lifetime" gas used, in gallons (doesn’t work if the MPG has been reset). And using the window sticker mpg rating or, better, your own Volt’s "gas mileage" number you calculate from recent gas miles /gas used (best if you drive far enough to use 1 gallon or more), you can multiply that "lifetime" gas used number by the gas mileage estimate to obtain an estimate of lifetime Gas Miles. Using the odometer reading then, math gives you the total ev/gas miles breakdown.

The "lifetime" number of gallons of gas used, by the way, is also the number of ev miles you need to drive without using any gas to have the MPG increase by 1.000.

The MPGe number, on the other hand, includes only the kWh of electricity put into the battery from the wall plug. The energy content of regen isn’t counted as electric fuel in the MPGe energy-equivalent mileage calculations. Any Volt driver who spends time driving up and down hills will accumulate downhill regen powered distances (doesn’t get counted as electric fuel used) and downhill distances propelled by gravity, not by electricity. A "mileage" number that doesn’t include all the fuel used to drive a recorded distance is a meaningless number, especially when distances achieved without using any fuel at all are included in the total.
The OP gives his "lifetime MPG" as being 64 and 70 mpg. There is no way that is just based on gasoline fired miles. Does your Gen 1 Volt ever get that high gas mileage when using gas? (that was rhetorical). To get that high a number, the computer must be averaging in MPGe.
 

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Anything is meaningless if you don't have a use for it. It's excellent, for example, to make valuable guess as to how many of the odometer miles were gasoline-fired versus on battery.
You can make an estimate, but you can't make a very good one. In order to make the calculation, you'd have to know or guess: (1) average MPG when gasoline-fired (which can vary by at least 20% based on temperature, speed, driving habits, road condition and tire pressure) and (2) average MPGe when using battery (which can vary even more than MPG for the same reasons). Because the car was used, the OP has no idea about how the original owner drove it. At most it will give you a rough idea whether the car was an "apartment car" (i.e., almost never charged and driven primarily as a gasoline-fired hybrid), a "grandma car" (i.e., driven on short trips almost exclusively on electricity), or somewhere in between.
 

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Discussion Starter · #10 ·
Danno, it is not using "MPGe". Lifetime MPG is simply taking all of the miles driven (i.e. the odometer reading) and dividing it by the number of gallons of gasoline the car has burned. You are inferring some other complexity into the situation (such as MPGe) that simply does not exist...
 

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Discussion Starter · #11 ·
now if you were trying to calculate the exact number of miles driven solely on gas (or electric), some estimation of other voodoo would be required since you don't know how many mi/kW were consumed in electric mode nor MPG efficiency while on gas.
 

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Your initial numbers make me wonder if the previous owner ever used the car as an EV and how heavy footed was his driving.
 

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Discussion Starter · #13 ·
obermd,
What is surprising (to me) is that trip B has not been reset since before the lifetime MPG was reset. My trip B shows about 12k miles of the car's pre-reset life. It appears that the prior owner was fairly electric (about 150 MPG in the 12k miles)
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Danno, it is not using "MPGe". Lifetime MPG is simply taking all of the miles driven (i.e. the odometer reading) and dividing it by the number of gallons of gasoline the car has burned. You are inferring some other complexity into the situation (such as MPGe) that simply does not exist...
That makes sense. Thanks for that explanation.
 

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You can make an estimate, but you can't make a very good one. In order to make the calculation, you'd have to know or guess: (1) average MPG when gasoline-fired (which can vary by at least 20% based on temperature, speed, driving habits, road condition and tire pressure) and (2) average MPGe when using battery (which can vary even more than MPG for the same reasons). Because the car was used, the OP has no idea about how the original owner drove it. At most it will give you a rough idea whether the car was an "apartment car" (i.e., almost never charged and driven primarily as a gasoline-fired hybrid), a "grandma car" (i.e., driven on short trips almost exclusively on electricity), or somewhere in between.
Yes, those things can vary, but for MOST CARS, driving under MOST CONDITIONS, taken in aggregate over years, they're gonna end up pretty close to the EPA estimates. Because the EPA does this A LOT and has a very good idea how cars consume energy across the whole US. That's part of their mission.

Plus, you're doing this math because you want to know the answer it gives. If you KNOW the math is wrong, you also know what the better answer is already. If you don't KNOW it's wrong, it's right enough to tell you more than you know already.
 

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now if you were trying to calculate the exact number of miles driven solely on gas (or electric), some estimation of other voodoo would be required since you don't know how many mi/kW were consumed in electric mode nor MPG efficiency while on gas.
But the EPA ratings give those. Are they gonna be right for an individual car on an individual trip? Probably not. But over a few winters and summers and being used in all kinds of conditions, EPA estimates are pretty good.
 

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The OP gives his "lifetime MPG" as being 64 and 70 mpg. There is no way that is just based on gasoline fired miles. Does your Gen 1 Volt ever get that high gas mileage when using gas? (that was rhetorical). To get that high a number, the computer must be averaging in MPGe.
"Mileage" terms can be unclear when the use of gas is optional ("mpg" in lower case can be ambiguous). My understanding is:

MPG = total ev miles + total gas miles / total gas used (lifetime or just for any trip)
MPGcs = total gas miles / total gas used (gas mileage in Charge Sustaining mode)
MPGe = total miles achieved using 1 Ge of fuel

Miles per gallon equivalent compares the energy content of fuel, where 1 Ge = the energy content of 1 gallon of gas = the energy content of 33.7 kWh of electricity.

MPGe is usually used to refer to the mileage during the Volt’s Electric Mode driving portion of a trip, but energy content is also consumed during the charge sustaining portion of a trip and can also be expressed in MPGe terms. One gallon of gas = one Ge of gas, so that my 2012 Volt’s window sticker could read either 37 mpg or 37 MPGe.

Quantities are customarily measured at the point where they are purchased, i.e., at the gas pump and at the wall socket. Gasoline loses no energy content as it flows from the gas station tank into the car’s gas tank. Electricity, on the other hand, flows through the Volt’s charging circuits to be stored in the battery, and some of those 33.7 kWh of "from the wall" electricity are consumed as charging losses when the battery is recharged.

For example, the Gen 2 sticker rates the car at 53 ev miles/charge, 106 MPGe, i.e., one full charge = 0.5 Ge of fuel. When recharging a fully depleted battery, 0.5 Ge = 16.35 kWh is pulled from the wall, 2.35 kWh of which become charging losses, and 14.0 kWh are put into the battery as usable grid power. 53 ev miles on 14.0 kWh Used from the battery = 53 ev miles on 16.35 kWh used from the wall plug = 53 ev miles on 0.5 Ge of fuel = 106 MPGe.

The Gen 2 Volt’s energy usage display can be configured to show (since last full charge):

Electric Mode MPGe = total trip ev miles / total electric Ge used (including charging losses), and
Extended Mode MPGcs = total trip gas miles / total gas Ge used
Lifetime MPGe = lifetime ev miles + lifetime gas miles / lifetime ev Ge used + lifetime gas Ge used
 

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nodnod MPGe are calculated perhaps foolishly, but they're based on the idea that gasoline has the energy equivalent of 36 kwh, and it can be perfectly converted. So the "MPGe" is "how many miles can be expected from 36kw delivered from the battery" without any regard for how many charges it may take or how much loss there is before it is IN the battery.
 
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