What's under the hood is the start of the divergence: the HS 250h gets its own version of Toyota’s Hybrid Synergy Drive hybrid system, combining a 147hp, 2.4L four-cylinder Atkinson-cycle engine with a 40hp electric motor system, together making 187hp. That gives the HS the oomph to get to 60mph in just 8.4 seconds, about 1.5 seconds quicker than the Prius.
That leads to less efficiency, of course. EPA fuel economy ratings are 35mpg city, 34mpg highway - not nearly as green as those for the Prius, but off the charts for an entry-level non-diesel luxury sedan.
As with most hybrids, however, real-world conditions proved a bit less forgiving than the optimistic EPA ratings. We saw about 30mpg keeping with fast-moving Orange County traffic, then in a separate loop, driving the HS aggressively on a short 12-mile stretch of hilly roads, mileage dropped below 22mpg. In exceptionally gentle, controlled conditions on level roads, the HS eked out 46mpg - about the same you'd expect from the Prius in normal, with-traffic driving.
The root cause of the difference in efficiency between the Prius and HS 250h isn't just under the hood, however. At 3,682lb, the HS weighs a whopping 600lb more than the 2010 Prius and about the same as the bigger Camry Hybrid - which, by the way, can out-accelerate the HS.
All that weight comes from somewhere, and a quick perusal of the spec sheet shows a fair bit of it is technology - the kind of tech that drives gadget fiends wild. In addition to LS-like high-tech features such as Intuitive Park Assist, a new heads-up display, Dynamic Radar Cruise Control, front and rearview monitors, and Lane-Keep Assist, the HS 250h also adds the very slick haptic joystick control for the multimedia/navigation interface we told you about in our 2010 Lexus RX first drive.
Overall, we liked the 2010 Lexus HS 250h for its well-built interior and quiet ride, which it owes to details like an acoustic windshield and triple-layer door seals. Some coarse engine noise did leak through under acceleration, though - more than some buyers would expect in a Lexus.