Ask the mother of a teenage driver what piece of emergency equipment they want their kid to have with them in the car and mom will answer "cell phone." Indeed, the days of carrying tools, jumper cables, and a jerrycan of gas in the trunk are over – just call AAA.

But still, the question beckons: What happens if you're hypermiling in your Prius and you overestimate your skill? You can certainly run out of gas in a hybrid just as easily as you can in any car – and doing so is just as stupid. "But wait," you say. "There’s that big battery! Can’t it help me out?"

Well, that depends.

Dead In Their Tracks

Chevrolet's Tahoe and Silverado hybrids, for example, will not operate without gas in the tank. That’s right, they completely shut down if they run out of gas – no limping to a gas station on the battery, even if it’s fully charged. We know this because we tried it, thankfully when we were equipped with a three-gallon tank of spare fuel.

GM's official word is that driving these big hybrid trucks without gas leaves the large propulsion battery in vulnerable state, so rather than risk damage, the vehicles shut down. GM is not alone in this strategy, as Nissan's Altima Hybrid will also shut itself down the moment the gasoline engine runs out of fuel, lighting up a triangular warning lamp on the instrument panel.

Prius Survival Guide

Toyota's Prius, as well as the similar Lexus HS 250h sedan, will run on battery power even if it runs out of gas, but not for long. How far it can go is directly related to how much of a charge the battery has. With a strong charge prior to the gas tank hitting empty, some owners have reported being able to drive up to two miles without gasoline. When the battery becomes discharged enough that the gasoline engine would normally fire up, the Prius' speed will be limited to 18 mph, says Toyota. So on your limp to the gas station, you’ll be going pretty slowly.

By this point the voltage of the main battery, which can supply up to 650 volts to the drive motor when it is fully charged, has dropped so far that the drive motor is on the verge of drawing too much current. (Remember Ohm's law from high school physics?) When this eventually happens, the system shuts down the electric motor and the Prius stops where it is.

Since there is no conventional "neutral" in the transmission, pushing a dead Prius will be more difficult because the electric motor/generator system is also being turned. Which leads to a bit of a problem: If the car is flat-towed by a tow strap, it will be generating electricity and you risk overheating the motor. So, make sure your out-of-gas Prius only gets towed with the front wheels off the ground.

There's another problem to be dealt with when you run your Prius to a stop, and that’s getting it restarted again. When the main battery runs too low to spin up the gasoline engine, adding fuel isn't going to do any good. Charging the nickel-metal-hydride battery takes a special Frankenstein machine that exists only at Toyota dealers. Toyota, however, says that so far no dealers have reported ever needing to use the machine since the 2001 introduction of the car in the U.S. Still, you don't want to be the first.

Ford's Fusion Hybrid uses a similar strategy as the Toyota and Lexus systems, and will allow about a half-mile of driving on the propulsion battery after the Fusion runs out of fuel.

Stupid Journalist Tricks

We decided to experience what some daring Prius owners have reported, and drove a Lexus HS 250h hybrid, which has a similar driveline to the Prius, until it ran out of gas. We picked a non-highway loop with some mild hills and drove quickly enough that we were able to keep the main battery fully charged, but not too quick that we used battery power to augment acceleration.

When the trip computer alerted us that our distance until empty was zero miles, we kept driving. And driving. Two hours and about 55 miles later, the HS 250h finally ran out of fuel. At first it wasn't noticeable, except that the car was quieter. However, after only half a mile, the car became significantly slower, and wouldn't accelerate beyond 15 mph. We weren't yet in danger of killing the main battery, and so we added some gas and the car was back to normal.

How Low Can You Go?

Some owners of Prius' have reported that they can drive up to 100 miles after the low fuel light comes on. The big problem is that the remaining range is not always accurate. Trip computers that display cruising range cannot forecast the future, should driving conditions change.

You can figure, however, that with an 11.9-gallon fuel tank and a 50 mpg combined fuel economy rating, the range of the Prius is nearly 600 miles. Not only is this well beyond most vehicles on the road, but that's at least a good eight-plus hours in the saddle. There's really no excuse for not stopping for gas – or at least topping off the tank during a bathroom stop.

Editor’s Note:

Here's another warning: Don’t drive your Toyota hybrid for too long on empty, or you’ll eventually need to get towed to the dealer. Toyota gave us some more information about how its hybrids function once they’ve run out of gas after we’d published this story. It turns out we were lucky we didn't deplete the battery of our Lexus HS250h any more than we did when we drove it with an empty fuel tank. All Toyota and Lexus systems are programmed to attempt to start the gasoline engine when the battery reaches its “recharge threshold.” If the engine does not start after three attempts, the system shuts down and requires a technician to reset a fault code before the vehicle can get moving again, even if fuel is added to the empty tank. There is no way for a driver to know when the battery charge gets low enough that this automatic shut-down occurs, but it’s a necessary precaution to protect the battery pack from harm.