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."
