Last week, the U.K.’s Advertising Standards Authority (ASA) stopped Porsche from running an ad claiming that the new Cayenne S Hybrid “can even travel to a speed of 37 mph without using its V6 petrol engine.” The agency believes the 37 mph claim is misleading because the vehicle travels only for a limited time and distance using power exclusively from the electric motor and batteries. The ASA is right. Hopefully, the ruling will raise awareness that the ability for conventional hybrids to travel in “all-electric mode” at high speeds is a cool trick—but relatively insignificant.

Porsche is not the only hybrid maker trumpeting the all-EV capability of its conventional hybrids. Ford promoted the Ford Fusion Hybrid’s ability to travel up to 47 mph without using gasoline. The 2010 Toyota Prius’s “EV” button allows drivers, under some conditions, to force the engine off, allowing the car to run up to 25 miles per hour from battery power alone for about one mile.

As any driver of a full hybrid—or a plug-in car—will tell you, it’s a magical feeling when a vehicle moves down the road without gasoline in near silence. But in the case of a conventional hybrid—rather than the plug-in kind—the net result of maximizing the EV mode is to quickly drain a battery that was not designed for high demand all-electric driving. Just like when you need to catch your breath after a long sprint, the hybrid’s gasoline engine is more likely to come on stronger and longer to refill that battery to an optimal level. In the end, that undermines the hybrid’s core efficiency strategy of juggling different aspects of the drive cycle with the best mix of gasoline and electric power. The mixing and matching of electricity and gasoline is how a vehicle like the Toyota Prius can achieve a remarkable 50 miles to the gallon.

Efficiency vs. Electricity

So, it’s a nice claim to say that a hybrid can travel X miles per hour all-electric, but the likely net result of high-speed EV mode is reduced MPG. This once again proves that the name of the game is efficiency, not electricity.

By the way, Porsche also says that the Cayenne hybrid can “sail” on electric power at speeds up to 97 mph.

The empty Porsche claims underscore that if you’re driving a non-plug hybrid, all of the energy to move the car ultimately comes from the gas engine—and all-EV mode is not going to last very long. This doesn’t mean that a hybrid’s all-electric mode is lame. Not at all. For example, it’s incredibly useful when traveling short distances at low speeds. That’s why Praveen Cherian, Ford’s hybrid program leader, speaking about the Ford Fusion Hybrid said the EV mode in conventional hybrids “allows drivers to maximize fuel efficiently in many driving situations. For example, this would allow drivers to travel around their subdivision and parking areas in all-electric mode.”

But the claim that a hybrid can travel up to 30 or 40 or 50 miles per hour in all-EV mode, even if true, is mostly a clever marketing claim that should be taken with a grain of salt.