How green is this hybrid?

Lexus pitches its LS 600h L as an earth-friendly alternative to long luxury sedans from German makers, which typically have EPA combined fuel-economy figures in the 15 to 18 mpg range. Lexus claims an EPA combined rating of 21 mpg for its hybrid übersedan.

Hang on, though. Such comparisons are tricky right now. That 21 mpg is actually 1 mpg less than the rating on Lexus’ own 2007 LS 460 L, the same chassis with a meager 380-hp V8 and rear-wheel drive. But—and this is key—the EPA rating system changes for 2008 and can’t be compared to any 2007 model’s rating. The LS 600h L is rated under the new system, which was devised in part to correct the overestimation of hybrid-car fuel economy. But we don’t know the ’08 ratings on the competitive vehicles, or Lexus’ own LS 460 L, so it’s not yet apples-to-apples.

Among the changes made in the pursuit of more accurate fuel-economy ratings is that the ’08 test cycle includes highway speeds up to 80 mph (the old one was devised back in the days of the national 55-mph limit). EPA says you can expect a non-hybrid model’s rating to go down, perhaps by 10 percent or more. So the situation is likely to flip-flop in ’08, and Lexus may be able to claim that its hybrid will get better mileage than the gas-only model.

Or maybe not; the LS 460 L is rated 22 mpg combined, already pretty efficient for a big luxury car, thanks in part to its eight-speed automatic transmission. The advantage for the hybrid, if any, could be small. A little elementary math suggests that the payoff in saved gasoline for your $20,000-plus investment in the hybrid technology could be . . . well, it depends on the price of gas and the final fuel-economy figures for the 2008 gas model, but not-during-this-lifetime is a fair estimate.

Of course, EPA ratings on competitive vehicles also are expected to decline for ’08. Comparing the LS 600h L to those cars makes the case for the Lexus as the green alternative among luxury sedans producing more than 400 hp.