I was always shocked by the 'good' mpg US hybrids get, compared to Uk/Euro diesels - (though some of the difference is down to different sizes in gallons between us, but still. My 12 year old volvo v70 (not a small car) got 53mpg easily
But are those UK or US gallons? Cars in the UK get 20% more miles per gallon simply because gallons in the UK are 20% bigger.
Prius highway rating of 4.4 L/100 km (53 mpg-US / 64 mpg-imp)
Volvo V50 highway rating: 3.8 L/100km - this is the 'extra economical' diesel, with start/stop, aero body and aero wheels, so a good match for the Prius - seems odd that all that extra tech wouldn't give them much improvement? I guess city driving it may be better but I don't have time to dig out the figures now.