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I just leased a new 2018 Volt LT. Great car! I went through the tutorials this am, and then took a 24-mile ride, mostly in city traffic, but about 10 miles of freeway, too. I zeroed the trip counter at the beginning of the journey and when I got home it was showing 250+ mpg average for the trip (this is in Driver Information Center display). I'm assuming that 250 is as high as it goes. But then as I got out of the car, the infotainment screen showed that I'd used 6 kWh for 24 miles.

I think the EPA calculates 33.7 kWh as equal to a gallon of gas. By that logic I used the equivalent of .178 Gal to drive 24 miles, which means that I was getting 135 mpg.

So am I reading the display on the dash incorrectly? Why did I get such a high value?

Many thanks to anybody who can shed light on this!

Steve
 

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the 250 plus refers to gasoline miles per gallon. Total miles driven divided by the fuel (gas) used. There dash also should have also shown MPGe witch is the 135 mpg you calculated. the MPG value is meaningless until you start using gasoline.
 

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Discussion Starter · #3 ·
Ah, that makes good sense. Thanks much. I finally found the MPGe display on the infotainment screen and it showed 117. I don't quite understand the discrepancy -- seems like it should have been higher. But 117 is fine by me.
 

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The concept on Wikipedia’s "Miles per Gallon Equivalent" page is that the energy content of the 1 gallon of gas you pay for at the pump is equal to the energy content of the 33.7 kWh of electricity you pay for and pull from the wall socket. Gas loses no energy content as it is pumped into the car’s gas tank, but some of the electric energy you pay for is used up when the recharging circuits heat up as the battery is recharged (charging losses).

The Gen 2 Volt window sticker rates the vehicle at 106 MPGe and 53 electric miles, i.e., the car’s battery holds 53/106 = 0.5 Ge (a half gallon of "energy equivalent" fuel). You pay for 16.85 kWh of electricity from the wall, and by the time it gets into the battery, you’ve used up ~2.65 kWh, and have ~14.2 kWh of usable power.

For your driving today, in effect, after you drove 24 electric miles using 6 kWh from the battery, the car had traveled 24 electric miles using a total of ~8.65 kWh of electricity from the wall, which is equivalent to the energy content in 0.257 gallons of gas, or 93.4 mpg (vehicle rated at 106 mpg). Of course, the recharging losses are fixed until the next recharge from the wall, and become a diminishing portion of the total used as you use up the remaining ~8.2 kWh from the battery.

I have yet to read anything discussing the role of regen in MPGe calculations. If "energy content" of the wall plug electricity is used in the recharging circuits, surely the same must apply for regen, and there are charging losses associated with the regenerative braking system’s generator output as it recharges the battery. Since the amount of regen you create as you drive can vary, so can the influence of the energy content of the regen on the distance you drive on electric power, but I’ve never seen this discussed.

The significant distance you can drive on one unit of fuel (106 miles) distorts the impact of the numbers on the display shown for short trips. Driving a Gen 2 Volt 20 electric miles uses only around 3 cups of "energy equivalent" fuel. I doubt if my previous gas car’s gauges were accurate enough to provide me with a very reliable "miles per gallon" number after I’d used only 3 cups of gas. It seems reasonable to think the same about the reliability of any "miles per gallon" of "energy equivalent" fuel after using only 3 cups of the stuff.

It is unfortunate that we have no better reference terminology for Volt performance than gas-car terms because of the difference in driving distance per unit of fuel (1 gal vs 1 kWh). Using mpg (lower case) can be confusing because gas is not the Volt’s primary propulsion fuel, so it’s important to indicate what you mean by "gas mileage."

MPGcs (gas mileage when using gas, i.e., driving in Charge Sustaining mode) = total gas miles/total gas used.

MPG (all caps) = (total ev miles + total gas miles)/ total gas used, a meaningless term for a car that can move down the road without using any gas at all. If you switch into Hold Mode and drive your Gen 2 Volt to work and back a total of 42 miles using 1 gallon of gas, your MPG = 42 MPG. If you drive to work using battery power and back in Hold Mode, you use only 0.5 gallons of gas, and your MPG = 84 MPG. If you drive to work and back using the battery power only, your MPG = 250+ MPG because you use no gas at all... same route, choice of when to use gas, and not how efficiently you drive when using gas, determines the "gas mileage."
 

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Use the energy screen on the center infotainment unit rather than the one behind the steering wheel. It gives you the option to see your electric efficiency, fuel efficiency, and "MPGe Combined" (Average of the above two figures accounting for miles driven on each fuel type. Tapping the box with the combined efficiency figure will let you switch between your MPGe and MPG figures.

For most people, the lifetime or trip MPG in the screen behind the steering wheel will become useless unless they are exhausting the electric range on a daily basis.
 

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I have yet to read anything discussing the role of regen in MPGe calculations. If "energy content" of the wall plug electricity is used in the recharging circuits, surely the same must apply for regen, and there are charging losses associated with the regenerative braking system’s generator output as it recharges the battery. Since the amount of regen you create as you drive can vary, so can the influence of the energy content of the regen on the distance you drive on electric power, but I’ve never seen this discussed.
I've noticed regen reduces my KWh use numbers (as long as I'm running as an EV). This has the immediate side effect of raising the MPGe numbers in my 2017 Volt. When running in any charge sustaining mode (Hold, Mountain, battery depleted) the car shuts off the fuel injectors until it needs the ICE again, effectively "freezing" the gas used number during regen. This raises the gas MPG value by increasing miles driven by not gallons used.

It's apparent from this that while there is no or very little public discussion about how regen impacts efficiency the powertrain engineers at GM had those discussions.
 

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The Gen 2 Volt is rated at 106 MPGe, or 106 miles of distance on the electrical equivalent of the energy content of 1 gallon of gas. The energy content of 1 gallon of gas = the energy content of 33.7 kWh of electricity. The 106 mile rating appears to be related to the quantity of the energy content of the grid power stored in the battery. Regen electricity has the same energy content as grid electricity, but the quantity created and used varies from trip to trip.

The kWh Used number shown on the Volt’s energy usage screen is not a meter reading, but a net calculation: grid power used less regen put back into the battery. In real life driving, each time you slow down or move downhill as you drive, you add regen back into the battery (you may see the used number decrease as you drive down a long hill). The energy usage screen may show "14.2 kWh Used" when fully depleted, but that’s only the grid power used (whose relationship to the 16.85 kWh from the wall plug is known). It does not account for the energy content of the regen created and used during the trip.

Battery-powered driving using regen obtained while driving in CS mode will, of course, be used to increase the gas miles. Use of regen, in a sense, inflates your gas mileage and grid electric mileage numbers by adding distances to the totals driven that were not done using that fuel.

The MPGe number shown on the Volt energy usage screen, on the other hand, should reflect the distance you drove using the energy content of all the electricity you used to drive in CD mode, including grid and regen electricity, and including any charging losses.

This it does not do and cannot do if the amount of regen "charging losses" are unknown. If 33.7 kWh of grid electricity from the wall - 5.3 kWh of charging losses = 28.4 kWh of usable power in the battery, then 33.7 kWh of generator output - ? kWh of charging losses = ? kWh of usable regen in the battery.
 

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There's a reason it's on the infotainment screen. It's for entertainment only.
 
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