You sure about that? Because I easily get about 70 mpg on my Prius (C class) when I'm supermiling in the city. Granted it's not as efficient on the highway, and unfortunately I do have to do a lot of highway travel. I can sustain a good a 55mpg average at the end of the month though. I plan to start borrowing my wife's whenever I'm on the highway, I think this way I can average mpg in the 70s overall when I do this.
The number for a Prius-hybrid you'd really care to look at would be the city driving numbers as that's where you get all the advantages of regenerative breaking. Running a Prius at 100+ km/h it just turns back into a standard petrol fueled vehicle carrying drained batteries.
I've found diesel engines and vehicles to be considerably more reliable and therefore less expensive. Fuel is between 5-7% more expensive here in the UK so things are good there.
The city driving numbers are about the same I found. I regularly drive right into London's congestion charging zone - at least 1-2 times a week.
51 is the expected MPG in city driving scenarios. I can achieve that with no particular effort in London traffic. I don't drive like a crazy person and hang back regularly - that is all.
My vehicle doesn't have piles of expensive electronics, batteries and things to go wrong either (look at recent threads about Toyota stack overflows for example).
The most expensive thing on a Diesel that can go wrong is the particulate filter, EGR valves and timing chains and these are cheaply regenerated and replaced respectively.
Bear in mind I drive a 6 year old 1.3L diesel Fiat van converted to a people carrier which is as aerodynamic as a shed. It also has 3 children, a buggy and a pile of crap in it.
Perhaps a computer could optimise this better but I doubt it. The computer and automatic box just goes when you push the pedal down. The driver isn't connected to it and feeling what it's doing and understanding the power curves.