I decided to fill the tank today, even though it is more than half full. The tank holds a little over 9 gallons of fuel, and I put 3.072 gallons it in to top it off. Normally, I would probably not keep a full tank in the car unless I was planning to go on a trip, but I just wanted to top it off to start tracking fuel consumption. In all, the car traveled 554 miles since I got it (with a full tank), so that’s a little over 180 miles per gallon when doing the straight calculation. Of course, for a more accurate picture, the electricity costs of charging must be factored in. The easiest way to do that is to take the cost of recharging and figure out how many additional gallons of gas that would have purchased and then do the math again.

According to the Volt itself, it is using 31 kwh of electricity per 100 miles driven in all-electric mode. On that first tank of gas, I had 448 electric miles and the balance (106) in gas-powered miles. So when running on gas only, I got 34.5 mpg. When running on electricity, it used 138.88 kwh (at 31 per 100 miles, multiplied by 4.48). Multiply that by $.09 per kwh and you get $12.50. The EPA says that 33.7 kwh is the energy equivalent of 1 gallon of gas (in terms of BTUs generated). That means that using 138.88 kwh is equivalent to burning 4.12 gallons of gas.

Now, add the 4.12 gallons of EPA gas to the 3.07 gallons I actually used, and you get 7.19 gallons total. Now divide the total miles driven (554) by the adjusted gallons used (7.19) and you get 77.05 mpge (miles per gallon equivalent), which includes the use of electricity to get a real equivalent to a gas-only car. It is the mpge number that should be used to compare with non-electric or hybrid vehicles, as that factors in the charging, where a straight simple mpg equivalent does not.

Better than a Prius? It depends. If you drive mostly highway miles where you would far exceed the initial 40 per charge and end up with a high percentage of gas-powered miles, the Prius would do better, at around 50 mpg vs. the Volt’s 35 mpg. But if you drive it like I do, on electric only 80% of the time, then my mileage, factoring in the charging costs, at least for this first fill-up, would be 27 mpg (more than 50%) better than the Prius. So it depends on your driving patterns. We’ll track this over time and see how this first fillup compares to averages in the long term.

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