Hybrid Car Fuel Economy Comparison

Current State of the Fuel-Economy Art
Mainstream car and truck buyers have demonstrated no real enthusiasm for fuel-efficient vehicles, but our government thinks we need them, so it's raising the corporate average fuel-economy standards for light trucks from the current 20.7 mpg to 21.0 mpg for model year 2005, 21.6 mpg for 2006, and 22.2 mpg for 2007. Car CAFE will remain at 27.5 mpg, at least for now. Higher-mileage vehicles are coming whether we want them or not.

Get an automotive engineer talking about fuel economy, and most will admit that, when designing a vehicle to meet certain size, weight, and performance targets, changing from a traditional gasoline engine to a smaller engine boosted by an electric assist motor and battery with regenerative recharging will net a fuel savings of about 25 percent. Switching instead from that original gas motor to a similarly powerful diesel engine also would save about 25 percent. In August 2002, we tested these claims of diesel and hybrid parity by sending a VW Jetta diesel along on a 250-mile mixed-driving loop with a like-sized Toyota Prius and Honda Civic Hybrid. Average fuel economy on the loop ranged from 39.4 mpg (Honda) to 42.3 mpg (VW), and acceleration to 60 mph varied from 12.7 seconds (Toyota) to 13.8 seconds (VW).

Of course, a diesel-powered hybrid would return even better fuel economy, but the diesel engine's future is in limbo in this country. Tough new emissions regulations threaten to outlaw light-vehicle diesels, barring a technological breakthrough that dramatically reduces emissions of particulates (soot) and NOx (oxides of nitrogen). These regs went into effect this year in states adhering to California's standards, and they'll be phasing in nationwide by 2008, which is why more carmakers are looking to gasoline-powered hybrids to boost their CAFE numbers.

Toyota and Honda have blazed markedly different trails into this hybrid frontier. Honda's Integrated Motor Assist (IMA) system in the Insight and Civic Hybrid is mild and elegantly simple in its operation--an electric motor is sandwiched between the traditional engine and transmission--either a five-speed manual or a continuously variable transmission (CVT). Fuel is saved by switching off the engine when stopped or coasting and by downsizing the engine and relying on the electric motor to assist with acceleration. The motor is powered by energy stored during deceleration and braking when the motor functions like a generator, recharging an onboard battery. (Today's hybrids are never plugged in for recharging.) Honda hybrids cannot accelerate on electric power alone.