Well it is difficult to get less than 45MPG (US gal) with a 3rd gen Prius and definitely takes the lead in the city. On the highway they are neck and neck. With hypermiling, both can get stellar numbers.
If only VW made a smaller version of the diesel for the US, they'd be an even better choice. Right now I think their smallest diesel is 2 liters for the USA and 1.2 liters in other countries (Europe?).
Chevy I believe is releasing a diesel version of it's Cruze to the USA for 2013. 2Liter engine I think.
When HCCI gas engines come out it'll be interesting too.
Here are the fuelly numbers in USA MPG for the 2012 Prius
http://www.fuelly.com/car/toyota/prius
I'm glad the US has efficient choices these days, the cars are out there and it's up to the consumer to sacrifice some power for some fuel sippage, and in some cases no sacrifice in power. Smaller engine but the electric motor makes up for it, as long as you don't need that power for a long time.
I got a link the other day about this guy who took a trip to Europe and was amazed at the fuel efficiency. He said he was in a wagon that got 53MPG on a 2,000 miles trip (Imperial I believe) and how they had cars over there that got 70+MPG. What he didn't realize is that these 70MPG claims are from the European test cycle and tend to be a tad overestimated, and 70/1.2 translates to 53MPG in US gallons.
The wagon, 53/1.2 is 43.5MPG USA.
He is complaining about not having these cars in the USA but the Prius V is out and has basically the same cargo space as a Subaru Outback wagon but gets 40MPG+.
http://video.staged.com/localshops/vw_passat_785_mpg_in_the_uk
I looked up the VW Europe site and the most efficient car (Polo bluemotion) they had was 80.7MPG on the combined Europe cycle, which is 67MPG US gal.
http://www.volkswagen.co.uk/#/new/polo-v/which-model/engines/fuel-consumption/
I am seeing that people tend to get 60MPG US gal (maybe more?) if they drive sensibly on the highway. So the bottom line is that a small diesel is more economical than a 1.8 liter Atkinson cycle gas engine in a 3,000 pound versus 2,400 pound car (Prius vs Polo).
I would love (and I'm sure others would) a 1.2L diesel car, as a mild hybrid of course or a plug-in (for guilt free city driving and extra getup and go).
I'm scared that some people think that the engines in cars in the USA are somehow less advanced and refined that engines in Europe and that is why their mileage is so much better. But rather it is due to small diesel engines, Imperial vs US gallons, the European test cycle and emissions controls.
Would a lot of people in the US buy low powered diesel cars? I'm not sure, and that's what the automakers are thinking too. But make it a hybrid with a good sized electric motor and then the only compromise is manufacturing cost.