Toyota Prius PHEV - click above for high-res image gallery

Ever since we started getting government estimates of fuel economy for new vehicles back in the 1970s, the first thing people noticed was that the numbers rarely reflected reality. The problem is that efficiency is highly dependent on the duty cycle and driver's behavior. However, in order to compare cars against each other a standardized test is needed, even if does not reflect drivers' own experiences. Over the years, the EPA has modified the test procedures to try to make the results more realistic.

When we got the news about the official launch of Toyota's Prius PHEV lease test program on Monday, we immediately went for the specs. In the footnotes, it became apparent that Toyota's range and efficiency estimates are based on the JC08 test cycle. JC08 is a new test cycle that is being implemented in Japan to replace the notoriously optimistic 10-15 cycle with the same intent. Unfortunately, it doesn't go nearly far enough. Read on after the jump to learn why.

[Sources: JAMA, Dieselnet]

While the numbers from JC08 are indeed lower than the 10-15 results, they are still wildly inflated. For example, the 2010 Prius is rated at 89.4 mpg (U.S.) on 10-15 a number that will typically only be seen by people like Wayne Gerdes, founder of On the JC08 test, the Prius is rated at a "mere" 76.7 mpg (U.S.). The EPA ranks the same car at just 50 mpg combined, a number in line with what most regular drivers will achieve.

Part of the issue lies in the test conditions. For the 10-15 cycle, the test starts with the car already warmed up, a condition which certainly does not apply to a great many driving events. When most people drive to work or drive home, they are beginning from a cold start, a condition which will initially consume more fuel. The maximum speed on this test is only 45 mph and the test runs for 11 minutes. For the newer test procedure, the duration is 20 minutes and includes both cold and warm starts. The top speed rises to 51 mph but the accelerations are still very low with speed ramping from 0-25 mph in about 10 seconds.

JC08 test cycle

Driving in standard hybrid mode with the battery plug-in charge depleted, the PHEV Prius is rated at 72 mpg (U.S.), slightly less than a standard Prius on the JC08 cycle. Given the optimism of that cycle, the PHEV will probably achieve somewhere close to the same 50 mpg as the 2010 Prius. It also seems reasonable to assume that it will fall somewhat short of the 14.5-mile electric range. Somewhere around 10 miles of electric range seems plausible and that would still provide a significant boost in efficiency to drivers who plug in every day and drive short distances. Just don't expect miracles.

While standardized tests provide a means for doing a head to head comparison between different vehicles, they can provide a real disservice to customers if they expect to achieve those same results without working hard to achieve them. Japanese authorities would do well to revise their test procedures further to try and make them even more realistic.

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