http://www.evworld.com/view.cfm?sect...le&storyid=887

the article talks about an interesting limitation that the prius has at higher highway speeds that effectively prevent it from being purchased in europe. i'll leave it up to you to read. the comments at the end are a bit technical but interesting.

from the article:

" Some time ago, Mercedes identified in public the 'Achilles heel' of the current HSD, namely the losses that occur at higher speeds, with the generator needing to supply power to the motor via the controller even when the battery is effectively out of the picture. This has been one of the reasons why DaimlerChrysler has joined with General Motors to develop an alternative which is more efficient at freeway speeds. "

from the author's comments:
" Of course the Hybrid Synergy Drive (to give it its proper name) is efficient at urban speeds, and also when running various governments' so-called 'highway' cycles (US average speed 48 mph); that's what it was designed to do. The issue is the increasing losses at steady cruising speeds, particularly at 70 mph and above. The problem is that the MG2 vector torque must be CONTINUOUSLY maintained during cruise, and the torque must be greater the higher the road speed, to allow increased engine torque to be fed through the single stage planetary gearbox to the road wheels. This can only be achieved by generating more electrical energy the higher the cruising speed, with the associated losses in the generator, controller and motor increasing as a consequence. Why else would they all need to be water cooled? A relatively trivial issue at urban speeds becomes significant on the freeway, as more than a few Prius owners (and Honda, Mercedes and GM) have all noticed. I am equally sure that Toyota has been aware of this limitation all along, and (correctly, in my estimation) judged it to be irrelevant in Japan and acceptable in California. Let's see if the Lexus GS 450h has an HSD, or something a little more suitable for Europe. 19/Aug/2005"