Ive owned several very efficient cars in the past, a Smart ForTwo Cdi, Citroen C1 HDI, and a Vw Golf Bluemotion, but my Jazz has proven to be one of the best for all round fuel efficiency.
Ive always used either Shell V Power or Tescos Momentum premium E5, and the best figure ive seen so far was 107mpg
The Jazz is simply not a car you can hurry or drive in a sporting manner, and it encourages you to drive efficiently, but the car regualry shows 80-90mpg, if driven very carefully
I even managed an impressive 78mpg on a long journey from N Wales to Cardiff and back, following the A470, hardly a road designed for fuel efficiency, but im very impressed with my cars economy