Hybrid Car Fuel Economy Real World Factors

Since then, urban congestion has worsened considerably. The average vehicle miles traveled per mile of urban roadway rose over 40 percent between 1980 and 1999, and the average delay per urban driver increased from 11 to 36 hours per year over roughly the same period. Highway speeds have risen at least 10 mph from the 55.8 mph average in 1975, increasing aerodynamic drag forces. New emissions test cycles have been developed to reflect some of these changes, but they don't affect the EPA fuel-economy test numbers.

Hybrids fare even worse in the real world, given that air-conditioning isn't used during EPA testing, and running the compressor to cool or defog the car places a disproportionately high load on a small hybrid drivetrain. Cold weather reduces battery efficiency and requires the engine to run more to warm the vehicle interior, which lowers real-world results. Very short trips penalize hybrids. And, finally, small-percentage test-cycle inaccuracies result in large drops in mpg figures when applied to the lofty hybrid numbers.

Several studies have suggested new correction factors for traditional cars and trucks, ranging from 16 to 20-plus percent, but none has yet to suggest a unique factor for hybrids, so we thought we'd take our best shot at it.

To quantify the real-world fuel economy drivers can expect from among the current crop of hybrid cars, we spent two days driving an Insight five-speed, a Civic CVT, and a 2003 and a 2004 Prius nose-to-tail around a city loop and on a highway cruise. Our city test used an 18-mile rectangular loop through Pomona and Ontario that was conceived by Southern California Edison as an urban test of electric vehicles. The highway test consisted of a run from Los Angeles to Palm Springs and back along I-10. At the beginning and end of each test, we carefully squeezed the fuel-pump trigger until the fuel remained level with the edge of the filler neck--a tedious five-minute procedure that ensures a repeatable before and after "full" mark.

Applying the same formula the EPA uses to achieve combined fuel economy, our mileage champ was the Insight at 59.8 mpg, followed by the 2004 Prius at 53.0 and the 2003 Prius at 51.7, with the Civic Hybrid well behind at 45.9 mpg. That the big, heavy, roomy Prius trailed the feathery Insight so closely and outperformed the Civic so decisively proves the value of Toyota's more complex full-hybrid approach.

To correct the EPA's raw figures to match our test results would require an 18-percent correction to the city and highway tests. To bring window-sticker estimates into closer alignment with customer-reported real-world driving would require correction factors of just under 30 percent to account for cold weather, air-conditioning, and congestion worst than what we encountered. Of course, even if the granite were re-chiseled today with these new correction factors, your results would probably still vary.