I read that the Volkswagon Polo Blue Motion gets about 62 mpg
using only a 4 cylinder turbo diesel engine in "ultraurban" mode.
The Polo Blue Motion is a 4 door family sedan (about the same
size as a 4 door VW Jetta) . The downsize to this VW
"ultraurban" mode is that acceleration is reduced to 17 second
timing for a 0-60mph acceleration (which is about twice as
long as a the acceleration of a most new cars in the USA). .
Still if one is driving in stop and go urban traffic this would be
an acceptable trade-off. The 2010 Toyota Prius and 2010
Honda Insight both have a economy setting that sound
really similar to the Volkswagon Polo Blue Motion's "ultraurban"
Which is fine as far as that goes, but if you look carefully at the
Prius design, you'll come to realize that a diesel engine would bring
nothing to the table with respect to being the ICE part of Toyota's
Hybrid Synergy Drive.