I was wondering how you could get as little as 45mpg from a Prius on shortish, low speed runs, but I suspect it's 45 miles per US gallon, which would be around 60mpg per UK gallon.
Hybrids require much more in terms of scarce resources to make them and will also use more fuel than the equivalent petrol car at speed over a long distance, since they will exhaust the battery support and the little engine will struggle with the load. I would have thought that small city cars would be better bets than large saloons for hybrids, however, the percentage weight increase and space lost to batteries would be more obvious.
Someone undertook a study of the cradle-to-grave cost of various new vehicles over 100k miles and apparently the basic Jeep is the most environmentally friendly, since it uses cheap, low-grade steels and other materials and has no air conditioning or exotic electronics. The extra fuel used is far outwwighed by all accounts by the environmental cost of extracting all those heavy metals for the Prius batteries.