I've got our V6 VN up to almost 40mpg @ 110km/h with the family and all our crap in it (on NZ roads). It was only doing 31 when I bought it... 4 speed auto. My worked 202 with YT head, XU1 cam, SU's, HEI blah blah blah, and 5 speed might get 23mpg with just me in it. The HJ is lighter than the VN, but the difference in economy is the EFI and the fancy chip and exhaust I've fitted.
Each car is different.. horses for courses.