As Ken points out, the 51 MPG is tested on the Japanese protocol -- in which the Prius gets around 80, so I would expect mid 30's city EPA.

R2-E2, 2G Prius.
Highway/City/Husband/Wife MPG: 56.5, as of 12/2005, 26K miles

Jac Nasser, Ford President: "We are planning to launch a hybrid version of
this car [P2000] within this year [1998]. We will also make FCEV available in
2004."