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Discussion Starter · #1 ·
Not a terribly important question, but I haven’t been able to come up with the pattern for MPGe increase/ decrease. Why does it increase/ decrease .2 sometimes and .3 other times?
 

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Varying driving conditions. Warmer, cooler, high humidity or low, high barometer or low, rain or not, wind or not, tire pressure, how much of a hurry you're in, how many traffic lights you catch or get stuck at... ALL those things will change day to day even while you're driving EXACTLY the same roads. Add in any variation in route and you can see multiple miles of MPGe change day to day. Even my lifetime MPG, one of the most stable numbers on the car, will go up or down a couple of miles between summer and winter.
 

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Discussion Starter · #3 ·
Not a terribly important question, but I haven’t been able to come up with the pattern for MPGe increase/ decrease. Why does it increase/ decrease .2 sometimes and .3 other times?
Thanks for the reply. I’m sure that all these factors affect the efficiency, but it appears the numbers are pre-set. If, for example my MPGe increases by a certain amount ( e.g. 79.7 to 80.0 or 80.9 to 81.1) and I then drive using gas, the decrease will always be to the previous number. The car wouldn’t go from 79.7 to 80.0 and then drop to 79.8. Nor would it decrease from 81.1 to 80.8. This math teacher has to figure out the pattern or just realize the numbers were randomly set.
 

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Somewhere...
Off in the distance...
I hear what I think is an engineer giggling about driving a mathematician crazy.
 

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Not sure what gauge you’re looking at to watch the MPGe increase/decrease as you drive on battery and then on gas, but that sounds like a "combined," or "lifetime" MPGe reading for a Gen 2 Volt. There is no MPGe display on a Gen 1 Volt. MPGe really is designed to describe an electric car’s "mileage" when running on battery as a comparison tool for the mileage ratings of various electric cars. It’s uncertain why one would otherwise want to calculate the total distance driven divided by the total gallons of "Gallon-equivalent" fuel used.

You did not mention your Volt’s model year. The Gen 1 Volts don’t even have a display showing any MPGe numbers. Is it possible you are speaking of the MPG figure?

As for the numbers that are displayed, I wonder if it is even possible for the programming to determine the MPGe data points with sufficient precision to adjust the increments in single digit steps using a single decimal place. More likely the numbers displayed are "rounded off" calculations. MPGe is describing how far the vehicle can be driven using one Gallon-equivalent of fuel from the wall plug, but the Volt’s electric fuel tank is not large enough to hold that much fuel. The Gen 2 Volt’s electric fuel tank holds only a half-gallon of Gallon-equivalent fuel, the Gen 1 Volts have smaller fuel tanks. Thus any MPGe calculations for the use of a portion of a full charge are based on a fraction of a unit of fuel, and quickly get into the realm of "rounded off" numbers. Combining that with "Gallon-equivalent" quantities of gas and gas miles to calculate a "combined" MPGe can lead one down a murky path. For example:

7 kWh Used will get ~26.5 ev miles at window sticker ratings from a Gen 2 Volt using 0.25 Ge of fuel

MPGe = 26.5 mi/0.25 Ge = 106 MPGe

and if you use another 1 kWh to drive an additional 6 ev miles, you get

32.5 mi/~0.2857 Ge = ~113.8 MPGe

and if you then drive 4.2 gas miles using 0.1 gallons of gas, you get

36.7 mi/(~0.2857 +0.1) Ge = ~95.151 MPGe

and if you then use another 1 kWh to drive an additional 6 ev miles, you get

42.7 mi/(~0.3214285 + 0.1) Ge = ~101.32 MPGe

If driving ~4 gas miles or ~6 ev miles can make the combined MPGe jump/drop by 5 points or more, one wonders how small a distance driven (or how radical a change in fuel mileage for a short distance) it would take to have the MPGe go up only from 79.7 to 80.0 and then drop to 79.8 instead of to 79.7.
 

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Not sure what gauge you’re looking at to watch the MPGe increase/decrease as you drive on battery and then on gas, but that sounds like a "combined," or "lifetime" MPGe reading for a Gen 2 Volt.
There's a per-charge MPGe reading on the Gen 2 Energy Usage display. I figured that's what was under discussion as it's the easiest thing to have presented and notice changing. Not my photo, but...

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Discussion Starter · #8 ·
That’s correct. This screen ( 108 MPGe) Lifetime, however, presents the ideal driving conditions. One of my nephews responding to a perceived change in my driving, said that I didn’t use to drive like a Grandpa. My 2017 Volt ( approx 26 K) shows 81.1 Lifetime. In the summer, I do get more than 50 mpg in gas mode. No one who lives in Portland, Maine and drives in cold weather could ever achieve this unless he/she swore off ever using gas. After all the analysis, my guess is that an engineer at GM set the numbers in .2 or .3 increments without any plan. These are, after all, estimates. Oh… the curse of having the mathematical mind and looking for numerical patterns in everything!
 

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There's a per-charge MPGe reading on the Gen 2 Energy Usage display. I figured that's what was under discussion as it's the easiest thing to have presented and notice changing. Not my photo, but...
Thanks for that photo. I note the 122 Combined and 108 Lifetime MPGe numbers are three digit numbers, i.e., whole numbers only (no decimal place). I do have some screen shots in my notes showing similar energy usage displays with Combined and Lifetime MPGe numbers lower than 100, and they do include a single decimal place, such as 81.2 MPGe. It’s difficult to know how precise some of the Volt’s displayed numbers are unless you see it.
 

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That’s correct. This screen ( 108 MPGe) Lifetime, however, presents the ideal driving conditions. One of my nephews responding to a perceived change in my driving, said that I didn’t use to drive like a Grandpa. My 2017 Volt ( approx 26 K) shows 81.1 Lifetime. In the summer, I do get more than 50 mpg in gas mode. No one who lives in Portland, Maine and drives in cold weather could ever achieve this unless he/she swore off ever using gas. After all the analysis, my guess is that an engineer at GM set the numbers in .2 or .3 increments without any plan. These are, after all, estimates. Oh… the curse of having the mathematical mind and looking for numerical patterns in everything!
The kWh Used on the energy usage display screen may be an "estimate" (actually, a net estimate of grid power used less regen put back into the battery), but MPGe numbers, based on data a bit more precise than the displayed numbers, are math calculations, not "estimates."

If your mathematical mind is looking for numerical patterns in the Combined and Lifetime MPGe numbers, why don’t you try calculating them using the info on the energy usage display?

Trip MPGe = Electric Miles / Ge Used
where 1 Ge = 33.7 kWh as measured at the wall plug

And

Combined MPGe = (total electric miles + total gas miles) / (total electric Ge + total gas Ge)
where 1 Ge = 33.7 kWh as measured at the wall plug = 1 gallon of gas as measured at the gas pump

The "iffy" part is converting "kWh from the wall plug" into "kWh Used from the battery" numbers. EPA rated the Gen 2 Volt at 106 MPGe / 53 ev miles per charge. That’s 0.5 Ge = 16.85 kWh from the wall plug to provide the Volt with the fuel used to drive 0.5 x 106 = 53 ev miles, Wikipedia indicates the Gen 2 battery holds 14.0 kWh usable, so 16.85 kWh of power at the wall = ~2.85 kWh consumed during recharging (charging losses) and ~14.0 kWh of usable power stored in the battery.

Thus, for Gen 2 Trip MPGe calculations, 14.0 kWh Used from the battery represents 0.5 Ge of fuel used, and each 1 kWh Used represents 1/14.0 x 0.5 Ge = ~0.0357 Ge.

The energy usage screen in hellsop’s posting above shows:

71.0 electric miles / 14.0 kWh Used = 142 MPGe
7.2 gas miles / ~0.138 gallons used = 52.2 MPGcs
(third decimal place added to the gas used to indicate unseen "rounding" of underlying data)

Total miles = 78.2 miles
Total Ge fuel used = 0.5 Ge (battery power, including charging losses) + ~0.138 Ge ( ~0.138 gallon of gas, no fueling losses) = ~0.638 Ge

Combined MPGe = 78.2 mi/0.638 Ge = ~122 MPGe

Try that with your own Volt’s energy usage display numbers to see if your Volt’s Combined MPGe reflects the trip ev and gas distance and fuel consumption data shown on the right of the usage display (within the limitations of the precision of the displayed data). Now consider that the Lifetime MPGe calculation is a similar math calculation based on lifetime data collected by the computer...

(Lifetime ev miles + lifetime gas miles) / lifetime ev Ge (total kWh Used, converted to total kWh from the wall/33.7) + lifetime Ge (in gallons used).

Theoretically, I suppose, one could use the Lifetime MPG and Lifetime MPGe numbers to deconstruct the Lifetime fuel used into Lifetime Gas Used and Lifetime grid power from the battery kWh Used, and use that to create a Lifetime miles/kWh Used number for the total ev miles driven...
 

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I've likewise noticed that my lifetime MPGe numbers are displayed to the tenths digit but only move in increments of 0.2, not 0.1.
 

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I've likewise noticed that my lifetime MPGe numbers are displayed to the tenths digit but only move in increments of 0.2, not 0.1.
Keep in mind that MPGe is not tracking the increase or decrease of quantities that can be measured in units or tenths of units (e.g., miles, fuel consumed). MPGe is tracking the ratio of the sum of two units (ev + gas miles) divided by the sum of two units (ev Ge + gas Ge).

Lifetime MPGe = (lifetime ev miles + lifetime gas miles) / (lifetime ev Ge used + lifetime gas Ge used)

As the car moves, only two of those four units will be changing at any one time (ev miles/fuel consumed or gas miles/fuel consumed).

The ratio of the two units that are changing (i.e., the fuel mileage of the fuel being used at the moment) may or may not be changing. Your gas or ev mileage may remain constant as you drive, but the total distance and fuel consumption will be increasing, increasing the portion of the total distances related to the use of that fuel and the impact that fuel's mileage has on the lifetime MPGe.

Fuel consumption rate doesn’t remain constant as you drive. When you drive downhill, the distances will increase, but you may not be using any battery power or gas at all. Mileage soars.

When you reach the bottom of that hill, you may then drive your Volt for some distance using the regen you created as you drove downhill. From the MPGe perspective, the distance driven on downhill regen battery power is distance driven without using any fuel at all (only grid power counts as fuel for MPGe calculations). If you regularly drive in hilly terrain, the downhill distances and the downhill regen-powered distances can measurably increase the distance driven totals without increasing the Ge used at all, boosting the MPGe numbers.

I would not expect the results of the calculation of MPGe to be displayed frequently enough to reflect increases or decreases in 0.1 increments.
 

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I would expect lifetime MPGe to change very slowly and it would be very easy to have the software display it to the 0.1 increment. Yet it doesn't. Strange.
 

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Plus a lot of the underlying stats are done in metric, so in addition to rounding and precision in the OBVIOUS places like screens, they're subject to rounding and precision and limit issues in non-obvious places too. That doesn't mean that the displays and readouts are wrong or not useful, just that ... one can't presume something is wrong or changing because of small variations of output without at least asking whether the output discrepancy itself doesn't mean what it seems to mean.
 
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