The iDSI will return about 600 km on full tank for mixed
urban/highway driving.
If you want to gain more on FC, go gentle on the pedal and keep it
in 'D' drive most of the time. If you are driving manual, keep the
rpm below 3200 rpm between gear change.
Never tested the new vtec version but heard that the FC is higher.
Power wise, only felt it after 5000rpm.