If there's one thing that seems to confuse people about the new Chevy Volt, it's fuel economy. What kind of mileage does the plug-in hybrid Volt get, and how does that compare to other cars, from hybrids like the Toyota Prius to electric vehicles like the Nissan Leaf? The answer is that there's no perfect way to compare the Volt to other vehicles. Even though some media outlets have reported observed mileage numbers -- like Motor Trend's 126.7 mpg -- these numbers are as meaningless as GM's original 230 mpg claim. That's because the Volt's gas mileage is entirely dependent on how far it is driven between recharging. So to go about comparing the Volt to other cars you need to be able to make an estimate of your typical daily mileage. Below about 40 miles per day, you won't need to use gas at all, provided you recharge the vehicle overnight. It's how far you go after you exhaust the roughly 40-mile electric range that determines how much gas you'll use in a Volt.

Lest we be accused of entirely ignoring the costs associated with charging the Volt, let's look at some numbers. According to the U.S. Energy Information Adminstration, the average retail cost of electricity for residential customers stands at about 11.5 cents per Kilowatt-hour (kWh). To completely recharge an "empty" Volt battery (which is actually only 65-percent drained, because the car always maintains some charge in the pack) means putting about 10.4 kWh of electricity back in. But since the process is not 100-percent efficient, it actually takes more energy than that, about 13.4 kWh using a standard 120-volt power outlet, according to Car and Driver magazine's testing. That's about $1.54, or a little more than the cost of half a gallon of gas at today's prices. If you were to plug in and recharge an empty Volt battery every day, you'd be spending $562 a year on electricity for the car. Not an insignificant amount of money, as it's enough to buy 200 gallons of gas.

But charging aside, what most people seem to be concerned with when looking at the Volt is how much gasoline they'd use, and how that would compare to other green cars. So we've done some math to make a comparison among the Volt, Leaf, Prius, and the forthcoming plug-in Prius.

Which ride is right for your commute? If you travel around 100 miles per day or less, it doesn't get more efficient than the Leaf and Volt, but the Prius starts to look better over longer hauls. Find further explanation below.
Adam Morath, Translogic

Let's start with the Leaf, as it's pretty simple to understand. Nissan says the Leaf can drive 100 miles per charge. Of course that means that it may go a bit further under ideal conditions, while it will also have a shorter range if you're driving it like an Indy car. But after you've exhausted its battery, that's it, you'll be waiting for it to recharge on a dedicated 240-volt charger, either one you've installed in your garage or at a public-use facility. While the Leaf is unquestionably the "greenest" car on the road, using no gasoline whatsoever, it's Achilles heel is its inability to drive past its 100-mile range.

The Volt shares the Leaf's all-electric ability, but it's limited to 40 percent of the Leaf's range. After that, however, the Volt's gasoline engine allows the car to travel another 310 miles or so before you need to fill its gas tank. The thing is, driving the car until its gas tank is empty without recharging as a regular practice would be foolish. You'd only wind up getting about 38 miles per gallon, which wouldn't make much sense, not when the Prius has a combined EPA fuel economy rating of 50 mpg.

But since most people don't drive hundreds of miles a day, the Volt will actually use less gas than the Prius, provided your average daily mileage is up to about 117 miles. That's because to travel this distance in the Volt, you will only need to use the gasoline engine for 77 of those miles. At about 33 miles per gallon -- a low-end estimate for the Volt's fuel economy when its gasoline engine is running -- it would use 2.33 gallons of gas. To travel the same distance, the Prius would use 2.34 gallons.

The wildcard in this comparison is the plug-in Prius that Toyota has promised for 2012. Since we haven't seen or tested the actual production version of the car yet, our math here is an even looser estimate than our numbers for the Volt, but this exercise should still give a good idea of what kind of daily driving range makes the plug-in Prius ideal. The Prius PHV (for Plug-in Hybrid Vehicle), as Toyota is currently calling it, is said to have an electric range of only 13 miles. The greater caveat is that you have to drive slowly to achieve that range, under 60 miles per hour, and if you push the throttle hard, the gas engine will kick on regardless of whether you have the car in EV mode.

So we have to make some assumptions for the Prius PHV. The first is that you can actually keep the car in EV mode for all 13 miles. The second is that a larger battery pack (which we're told adds 300 pounds to the car's curb weight, about a 10-percent increase) will cut the standard Prius' 50-mpg fuel economy a bit. We asked some engineers about this and were told to expect a 3.5 percent reduction in fuel economy, which would bring the combined rating of the Prius PHV to 48.25 mpg.

If we compare the Prius PHV to the Volt then, we find that the breaking point is closer to 100 miles. Again, the Volt would have to use its gasoline engine for only 60 of those miles, using 1.81 gallons of gasoline in the process. The Prius PHV would be using its gasoline engine during the last 87 miles, while burning 1.80 gallons.

Some conclusions are in order, but first, let's keep in mind that these numbers are all estimates, and as they say, "your mileage will vary." While the third-generation Prius is a fairly known quantity by now, nobody has even taken retail delivery of a Volt (or a Leaf, for that matter) and the Prius PHV is still an entire model year away from production.

That said, it seems pretty clear that as long as you're driving less than 100 miles a day, the Volt will use less gas than the Prius, the most fuel-efficient hybrid on the market, or its plug-in sibling. At more than 100 miles of daily use, the Prius PHV is probably going to be more fuel-efficient than the Volt. On a slightly higher mileage regimen, over 118 miles per day, the Prius is also likely a better choice than the Volt. And the Leaf? Well, provided you never want to drive more than 100 miles in a day, it will trump them all.

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