Steve Woodruff has created independently a PHEV Prius. You can view his work at his site(s):
-http://www.autobeyours.com/PriusParts.htm
-http://www.autobeyours.com/PHEV.htm
Step by step PHEV Prius conversion instructions(?) may exist at this site:
I]
http://www.eaa-phev.org/wiki/PriusPlus-Instructions[/i][/font]
I'm not an engineer or mechanic. It is time for our the "skilled" posters to chip in on this information.

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2005 AWD Escape Hybrid
Best
tank trip MPG 39.02 (scangauge II) for 402 miles on I-70, 10.3 gallons used over mostly flat terrain.
Best
tank trip MPG 34.6 for
E30 for 271 miles along I-80 in Indiana and Ohio.